Quote Originally Posted by 1351
The first exposure that shows a faint, light-gray tone is the optimal preflash time for my purposes.
Wow, thanks, that's really helpful information right there. I had imagined some nightmare scenario of simultaneously trying to calibrate both the paper's sensitivity for exposures plus the pre-flash settings and I figured it was going to be a long and painful process, but if all you need to do is find the exposure that gives the faintest impression on the paper... that's much easier. The circle makes sense so you can look for the border between the exposed and unexposed paper.

Thanks! I'll get this nailed down and then move on to trying to sort out exposure in the real world.