First, I think you need to determine what kind (ff or dacron)of strings you have on the two bows. If they are flemish twist as I assume they are, find a small strand protruding from the splice area and try to cut it with a pair of scissors. If it cuts, its dacron if it slides between the blades but wont cut, its a ff type material. If you cant do that measure the string off the bow pulled snug, then measure the string on the strung bow. If its dacron the difference might be 3/4" or more, if ff it will be near the same.
A dacron string might require a higher B.H.