I did the tests before flexible existed… In fact, I think castable had just came out very recently at that time. The method is described in the link below, but you will want to repeat one or two of my tests so you can determine a multiplier for your results. This will adjust for variables that are different for you.
ALSO - the first layer is drawn many times, the second is drawn only twice. I always photographed the second layer / set of laser passes. The huge amount of time it takes to do a peel cycle makes it easy to capture your photo without getting a third pass. So start photo during peel 1 and end photo during peel 2.