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Michael --
If you've arrived at an exposure that's giving you the "two clear steps" you're probably pretty close to setting the speed point on 2.4 — provided that you're using one of the film/dev combinations that appear to be honestly speed rated (TMY in DDX, TMRS, or Xtol, for example).
But the "2.4" target value is not carved in stone; it's significant only if you plan to use any of the film/dev data that I've published (or will publish). I recommend it simply in the belief that standardization is probably a good thing because it will make discussions and trouble-shooting a little easier. Also, 2.4 is a good compromise location because it provides adequate "toe space" while preserving enough curve length to make the Plotter's SBR estimations reasonably reliable.
But you can use any reference value that's convenient for you, and if, for example (while using one of the "honest" film/dev combos), you find that the "EV 2.0" value (for TMY) places your speed point at, say, 2.2, that's OK; just replace the 2.4 "default reference speed point" value in the Plotter's option screen with 2.2 and go with it.
What this example means is that your test conditions are different from mine — perhaps our meters are calibrated a bit differently, perhaps we're reading different "white" on the baseboard, perhaps you're using an incident meter instead of a spotmeter, perhaps our times are different, etc.
If this is the case, I'd suggest changing the EV value as necessary to place the calibration speed point on 2.4. In the example above, this would mean using EV 2.67 instead of 2.0 for 400-speed films, with similar 2/3-stop EV adjustments for films of other ISO ratings. Just be sure you go through this calibration exercise using one of the film/dev combinations that seem to be honestly-rated. You can scan the charts in my "Library of Film Curves" article to locate suitable film/dev pairs. Look for G curves that are very close (+/- 1/6th stop) to the film's nominal ISO rating at the G=0.62 level.
Does this clarify anything?
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