If I may make another point. According to Todd and Zakia, "The speed of a photographic material is not a fundamental concept, but provides an index number useful for calculating camera settings." Film speed isn't like miles per hour. It's based on the amount of exposure that will produce a quality print as defined in Jones, L.A., The Evaluation of Negative Film Speeds in Terms of Print Quality, Journal of the Franklin Institute pt 1, Vol 227, No. 3, March 1939, and pt 2 in April 1939.
The concept of quality is an interesting one. As Jones points out, "The problem therefore involves the consideration of subjective factors in addition to those of a strictly objective or physical nature." The tests involved a group of observers determining from a set of prints made from differently exposed negatives. They were to place them in order of worst to best. Jones found that after a certain point, there wasn't a noticeable change in quality. He chose the exposure that made the print that first exhibited a high degree of quality. This psychophysical test is frequently referred to as The First Excellent Print Test.
As part of the evaluation, a set of instructions is needed to help set the judging parameters. Jones considers the expression photographic quality as, "reasonable to assume that this aspect of photographic quality is dependent largely upon the fidelity with which the subjective evaluation of brightness and brightness differences produced in the mind of the observer when viewing the original is reproduced in his mind when he views the reproduction. However, it is not safe to assume, in all cases, that a precise reproduction of the original subjective impression is that which the observer would accept as the best possible photographic quality, for he might be more pleased with a reproduced subjective impression into which some distortion has been introduced."
From my interpretation, if the photographer's intention is to produce an image that does not intend to adhere to the practices defined in the first excellent test, then the very concept of absolute or correct exposure no longer applies.
There's also the concept of "quality" which varies from person to person and within the same person. This is why there are so many "systems" out there that seem to work. So, in effect, you can have a system that is technically wrong or mistaken in how photography functions, but still works. As with all natural phenomenon, one can think of it as a normal distribution curve or bell curve. Most will generally fall in the middle of what can be considered quality, and then it tappers off in both directions but never falls to 0. Think about disposable cameras or the old instamatics. They have a fixed shutter and aperture, but produced acceptable images most of the time. The whole idea of pushing for speed comes down to what is considered an acceptable result.
After determining the first excellent print and reviewing the technical characteristics of the negative that produced it, Jones then had to find a way to predictably and consistently reproduce that level of quality without having to perform the tedious psychophysical test with each new film or batch. He looked at all the previous speed methods including the popular inertia, England's 0.10 fixed density, and a 0.20 fixed density, but found that a speed point related to the slope of the curve produced a negative closest to the first excellent negative with the greatest number of different film types under the greatest number of conditions.
According to Jones, "From the standpoint of tone reproduction theory there seems to be no justification for the adoption of any value of density as a significant criterion of the speed of a photographic negative material. The primary function of the negative material is to record brightness (today called luminance) differences existing in the scene. Density, per se, has no significance as an indication of the ability of the photographic material to perform this function. The value of negative density by which any particular object brightness is rendered, as, for instance, the deepest shadow, is of no consequence except insofar as it may have some bearing on the exposure time required to make a print from the negative."
"Tone reproduction theory indicates there is only one characteristic of the negative curve that is significant in expressing the capacity of the material to reproduce brightness differences, and it is upon the way in which brightness differences are reproduced that the quality of the final positive must depend. This characteristic of the D-log E (today D-log H) relationship is the gradient of slope since this determines the magnitude of the density differences by which brightness differences in the object will be rendered in the negative and eventually in the positive made therefrom. The term gradient is used in the previous sentence not in reference to the slope of the curve a particular point, but rather it implies a consideration of the gradient characteristics throughout the entire used portion of the negative curve. There seems to be little doubt that the most logical and theoretically satisfactory solution of the speed evaluation problem lies in the development of a method by which the effective camera speed is expressed in terms of an exposure which is some function of gradient."
This became the Fractional Gradient Method where the speed point is found at a point where the gradient is 0.3 (0.3G) that of the overall gradient. For the record, this is not the point where the exposure is to fall. Part of the formula in calculating film speed includes multiplying it by 4 or in effect adding two stops. Before 1960, films like Tri-X and Plus-X had film speeds one stop lower than today. With the 1960's standard, the one stop safety factor was removed. Side fact, I believe the X in Tri-X, Super XX, and Plus-X represents the value of 50. Plus-X as 50, Super XX was 100, and Tri-X was 200. The one stop slower should also sound familiar to Zone System practitioners. In, at least, the first few editions of The Negative, Adams thanks C.E. Kenneth Mees from Kodak for his assistance. Jones worked under Mees and he was putting on the finishing touches to the Fractional Gradient Method at the same time as Adams was working on the Zone System. A 0.10 fixed density, while also popular at that time, could be used to approximate in may cases speeds similar to the Fractional Gradient Method which included the safety factor. However, when the safety factor was removed in 1960, the Zone System never changed. In effect, it still incorporates the safety factor which is why ZS testing generally produces speeds 1/2 to 1 stop slower than the manufacturer's ISO speeds. But as seen by the first excellent print test, extra exposure doesn't reduce quality (apart from adding grain and reducing sharpness, which isn't too big of a problem with large format). So, technically, the Zone System method is incorrect, but the results are satisfactory.
Some might say that the 1960's standards and all ISO standards thereafter incorporates a fix density method of 0.10. True, but only if the contrast parameters are met. C.N. Nelson, who was Jones' assistant, later found that there is a direct correlation between the fix density method and the fractional gradient method. The fractional gradient method was difficult to do and prone to errors because of the difficulty of finding the exact 0.3 gradient point. By following the contrast parameters of the standard, a speed value will be produced that will be identical to that of the fractional gradient method. For any conditions that fall outside of the parameters, an additional calculation must be made. The 0.8/log-H cannot be applied. In fact, when compared to the fractional gradient method, the fix method without the additional calculation will produce underexposure for contrasts greater than the standard and overexposure for those under. So, the fixed density point of 0.10 isn't an aim nor a goal, but a point of calculation. I've written a paper on this which should be somewhere in APUG titled, Delta-X Criterion.
In a comparison test I did for the paper, Plus-X had a speed of 125 for both the fix speed method and Delta-X method at normal processing. At around a +1 the fixed speed method produced a speed of 144 which rounds to 160 while the Delta-X method produced a speed of 129 which rounds to 125 or no change in film speed. The reason is that as the gradient of the film changes, so does fractional gradient point in relation to any density point. The original standard that used the fractional gradient method didn't state a specific contrast except that it had to be over a gamma of, I believe, 0.50. So in effect, when you consider the fractional gradient method, film speed isn't that sensitive to processing conditions. You could even say there is only one film speed to any film/developer combination.
I'm going to stop here because it is getting way too long. Hope this gives some perspective on speed.