Hi Doremus, as you know we've been through this in great detail in many threads. I started out, like many people, with the Zone System. When I eventually learnt a little about film speed, exposure, sensitometry, and tone reproduction, I realized many of the things we do when it comes to "calibration" are superfluous and/or misleading. It isn't
all bad, of course, but a lot of it is.
Re why so many Zone System practitioners advocate finding a personal EI, mostly tradition, combined with a lack of understanding of exposure/film speed and tone reproduction.
a. We are commonly told things such as testing for an EI yields a film speed based on how "you work". This is mostly incorrect. We are led to believe there are all sorts of processing differences which will lead to different film speeds. Actually the biggest variables are in metering and evaluating subject luminances. The standard Zone System EI test doesn't address these factors. Further, even if it did, differences in the ways each of us judge subject luminances are unlikely to be consistent. We are also sometimes told finding a personal EI compensates for things like aperture or shutter inaccuracies. Again, the test doesn't really address these. Unless you test for an EI with every lens on every camera at every aperture/shutter speed combination, under a variety of scene situations...
b. With the majority of modern films, used with properly formulated developers, the EI result of a Zone System test is predictable, and doesn't tell us anything new - unless your equipment is way out of whack. With few exceptions, people come up with EIs 1/2 to 1 stop lower than ISO speeds. Why? Because the Zone System methodology locates the speed point 2/3 stop below the ISO speed point. Why? Basically because it has a larger safety factor dating from when meters, shutters and emulsions were much less consistent/reliable.
c. The Zone System test is a no-flare test. Under actual photographic conditions, flare distorts the way low subject luminances fall on the characteristic curve. From a Zone System perspective (ie fixed density target for speed point), this means speed under shooting conditions will always end up higher than in a no-flare test, and contrast in the lowest "Zones" will end up lower than we expect. Flare is another very important reason why contrary to what we're sometimes told, a Zone System EI test does not give us more "realistic" film speeds for use under actual shooting conditions (we are also often told ISO speeds are "laboratory" numbers).
d. ISO speeds are rooted in tone reproduction and print quality. A fundamental concept is that the speed point has to do with contrast in that area relative to overall contrast. Zone System EIs don't do that since they are based on finding a fixed density speed point (usually .10 above fog). One interesting consequence of this is that when developing to higher or lower than normal contrast, Zone System EIs move more than exposure theory says they should
e. Related to the points above, Zone System calibrations tend to give us a false sense of the control and precision we can achieve when making negatives.
It follows from all of the above that barring extreme procedures/materials, there isn't much need to actually test for a Zone System EI. Rather than it being a test which somehow "reveals" new information, all it really does is confirm the difference in methodology relative to what's on the box. You can just skip the test, know that a Zone System EI will nearly always end up around 2/3 stop below ISO by definition, round that to 1 stop, and you're done.
Please excuse typos, etc.