I've tried using Stu Millers cal. and have always been stuck on how to measure the strike plate. I have used a micrometer, but with soft strike plates like Velcro and other soft strike plates do you barely touch the material or crush it down? (I have 3 velcros all different thickness and some soft leathers.)
In am in no way an expert, but in eyeballing the industrial strength Velcro, it looks to me to be about 1/8" so that would be 0.125 you would key into the calculator for thickness. But I could be wrong. Others may want to chime in here. 1/16" would be 0.0625
What meat says. Also, just put an arrow on the string and lay the bow on its side to see how much the arrow compresses the velcro. Measure it at that thickness. If you're off by a sixteenth of an inch or so, it really doesn't make much difference.
In the end it is the bareshafting, not the calculator that will make the decision on the correct dynamic spine...plus a softer material vs. a harder material will also have some effect that may be more important than the thickness...there are other factors, a BCY-X string for example, that have a far great effect that the calculator does not allow for...try to find a 24 strand string in the calculator
DDave