Ahem,
to do what you just explained, you must have previously done some testing to find the setting on the em10 that you want to reproduce. Therefore you will already know what the head height, the aperture and the print time to produce that setting on an EM10 are. Why do you need a bloody em10 if you already know that. Its just a useless crutch for the insecure.
Oh, and take some notes next time you print so you know what the values are.
We clearly approach the EM10 differently. I don't use it to make measurements of tones from a negative. I use it to measure light from the light source (or sometimes, through the film rebate), and then to replicate that light intensity. I think that having that ability to replicate conditions can help someone new. I know that it is particularly helpful in my context, which involves printing from multiple formats and films, using multiple lenses, sometimes changing diffusers in my multigrade head and sometimes printing in other people's darkrooms.
Using the contact sheet example, to use it effectively, you do need to first use trial and error to make one good contact sheet on the paper you intend to use.
Once you have that, you need to record the print time you used, and you need to measure the intensity of the enlarger light by nulling the EM10. Don't change the lens aperture - adjust the dial on the EM10 until the green light turns on on the EM10. Record the number on the EM10's dial.
From then on, if you use the same paper you
never need to worry about which enlarger you are using, the enlarger's height, which lens you have installed, what diffuser you have installed or condensers you are using, or what (empty) negative carrier might be installed.
Just adjust height and focus until the contact sheet area is well covered by the illumination, set the EM10 dial to the number you recorded and adjust the aperture on your lens until the EM10 turns green. That means the light at the contact frame is the same intensity as with your first "good" contact sheet, so you can use the print time you used. Sometimes the nature of the negatives causes me to do another one a bit darker or a bit lighter or with different contrast.
You can use a similar technique with normal prints. Just make a good first print (by trial and error) from a negative that prints well from something close to grade 2, and use the EM10 to record intensity through the film rebate (between frames). Record the printing time. The EM10 can then be used to match that intensity for future prints from different negatives, cropped and enlarged in different amounts. The time you recorded will be an excellent starting point for your test strips.
It works really well for making different size prints from the same negative. Once you get a good print at one size, without changing any settings on the enlarger measure the light intensity without a negative, adjust the enlarger height and focus, take out the negative and adjust the aperture for the same intensity as for the good print. The time(s) you used for the good first print will probably be really close to ideal for the new size. It helps to have more than one, easily replaceable negative carriers.
With respect to prints, none of this eliminates the need to make tests. What it does do is get you to a close starting point right away, which improves the quality and decreases the quantity of test strips required, especially for someone new to the darkroom.
I need to have a particular number for each paper I use. I usually don't need to worry about batch variation, because there is enough consistency within the papers that I use that variations are caught by the test print procedure.
