O.L. Adcock posted a formula here once. I cannot find my copy of it but I believe it was this:
(Bow weight times six) divided by 25.
O.L. considered 25 pounds to be the breaking strength of B50.
Chad Francis of Lost Creek Bows advised me to use a 14-strand B50 string on the 55 pounder I have and that is consistent with this formula.
Hopefully someone will second this motion or offer a contrasting point of view. I'm doing this by memory, so it may have to be taken with some skepticism.