I don't think there can be a set ratio between the string and cable. It would depend on how long each is in relation to the other. However, I've used strings from several of he better custom makers and they almost always come ready to go and have about a twist (complete revolution) per 1 1/2" of string.

You can add or subtract a twist or two to get proper peep orientation, but the cables can be twisted or untwisted more because it won't affect the peep.