Just a quick update on my progress. The calculation of exposure as about 1.1 lux s, and therefore the assumption to get flash output to f/1.0 at ISO 100 and then to reduce it by a stop or so by reducing the power level has proven quite accurate. I have run the pre-tests, for both 320TXP and HP5+ and the first 2 steps of the 31-step Stouffer are fb+f on 320TXP, while the HP5+ shows 5 steps of fb+f (bit odd to seem slower, reciprocity?). I have now exposed 15 sheets of each film, having slightly adjusted the exposure by -1/3f for 320TXP and by +1/3 for HP5+. Unfortunately, I have also experienced a spouse-related lights-came-on-in-the-darkroom issue (also known as a quite personal, don't-forget-to-tape-that-switch-to-the-wall issue), so I need to do this thing again... I am waiting for a bit more film, hopefully arriving later this week. Who said that film testing was bad for film manufacturers?
While I wait for the film, I am pondering if, and how, to improve my "sensitometer". I am not too skilled in manual construction, but I could consider making some sort of a reflective box or a diffuser for the flash head. I am also going to post a WTB for an EG&G.