Most of the initial stretching is from squeezing the wax out of the string, which is the same whether the string is made from B55 or D97. Initially, with a minimal amount of twists in the string, I like to see the brace height about 1” high, which I find works its way down to a normal brace height over a day or so with a hundred or so shots, and then continues to stretch for a few weeks until I add a normal amount of twists to the string. I have never really added any length to a fastflight string over a dacron string, although a dacron string may end up with more twists in it. But if you want to account for that difference, I would say no more than 1” longer, which if your string jig doubles back four times would be a movement of 1/4” of the peg.