B50 is THE stretchiest material out there. I just made a B55 string for my Super Kodiak, 60" AMO. My board is set for AMO string length. With B55, I have learned to go to a peg 3" shorter than what I want, given my twist method/style. Each string maker has their own style, just like an oil painting artist. Once you learn you repetitive style, strings become much easier to hit on the first try, but even that is no guarantee. I have a bunch of strings hanging on a peg on which I missed, usually by too long. It has been a while since I hung another on that peg, though.
Make personal notes of length peg used, loop starting point, loop length, taper length and # twists to get there, how many back twists, final loop size, etc., etc., etc...
Like Daniel said, learn with B50 or 55. Missing with other materials gets more costly.