• You may be making that too complicated. "Fitting" the step wedge exposure on the film is really by trial-and-error. For example the Wejex sensitometer has an adjustment pot. Instructions indicate to adjust it so the important part of the curve is evident. One would really need to adjust a sensitometer for each film speed, so you can get the maximum number of steps to plot, but I leave mine the same so I can do some relative film speed comparisons without touching the adjustment.
Also, the EG&G is 'adjusted' for each film type with ND filters. But again, I keep it set up for 400 film and let the slower films fall off on the end and don't get as many steps to plot out on the shoulder.

If you really want to try to figure it out with as much math and as little testing as possible you can also work backward from the ISO equation. In this case using iso 100 film. So, you want the 0.1 log d (speed point) to fall on , say step 17 which we will say to be 2.5 log d on your step wedge. You can add the densities if they are both represented as log (2.5log + 0.9log) = 3.4 log. By taking the anit-log of that you need 2512 millilux-seconds exposure on the step wedge.

Where did the 0.9 come from? That is the exposure needed to give 0.1 log density on the film and comes from the following relationship:
ISO = 800/(millilux-seconds) (by definition)
100 = 800/(millilux-seconds)
millilux-seconds = 800/100
millilux-seconds = 8
millilux-seconds = 0.9 log

The short of it is that I think Phil's number of EV4 is just a suggested starting point. Not part of the whole BTZS calibration parade.