I decided to make a chart of the difference in barrel length as the helicoid was rotated and compared this to the distance marks. I compared the 50, 100 and 150 mm to focus distances & went around in circles trying to make things line up...I then normalized one column of data to make them 'match' calcualted & measured, and scaled the f.l. to correspond, thinking there might have been an e.f.l. vs nominal. There were also problems extrapolating...it didn't convince me I had a wrap on it all, but I can see that there isn't much difference to reach infinity. As I suspected, I can't pick one single spacer to allow conversion from infinity focus spacing to the parfocal scale distances for all focal lengths, but eventually (after a few other experiments) I'll put it on a Speed Graphic anyway & be able to do whatever.

Thanks for the exercise.