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Michael --
The Expo/Dev program converts your Hi & Lo EVs and your Hi & Lo zone placements immediately to SBR, then (in zone system mode) divides the SBR into 7 equal parts and labels them as zones II through VIII. Zones I and IX are left outside this range to function as accent black and white.
If you want an approximation of the zone equivalent of any other area of the scene, read its EV and plug in into the "test EV" field. Of course this has to be based on the assumption that the film curve is linear, with zones of equal length—which is obviously not the case. But, as you say, given that there's great variation in curve contours (plus some other unforeseeable variables) it isn't practical to try to tailor these calculations to individual curves.
If you want an approximate negative density "equivalent" of a selected zone, assume your ES to be the film's DR and divide it into the 7 zones. Locate the zone or zone fraction density you're interested in, add the IDmin density, and you're probably fairly close. I don't know how worthwhile this information can be since each zone is a range of values, not a single density, but perhaps you can further apply some correction for a visualized curve non-linearity and come up with something useful. You can get a much more realistic idea of where the zones are going to appear in the negative (and the final print) by running your materials through the Plotter.
As far as zone choices are concerned, all calculations are done on the SBR so there's no restriction. Use any pair of zones (whole or fractional) you like. They'll all provide the correct exposure and development information, but I can't guarantee that they'll agree with the zone gray values visualized in the traditional zone system. But then, the zone system itself can't always do that, either.
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