As for split shafts, I measure about 3 inches of string beyond the post in question before I cut the strings for the wound strings on a Strat, but on someone else's advice I leave the plain strings full length for added tuning stability. You loose about 3/4 inch bending the string over and sticking it down the hole. 3 full wraps round an about a 1/4 inch diameter post requires 3 x pi x 1/4 = about 9/4 or 2-1/4 inches (about 57 mm). So the total extra length required is about 2-1/4 + 3/4 = 3 inches. I also found that with a little practice, it's usually possible to get the first and sometimes the second wrap by flipping the string over the post rather than turning the tuner. It save a little time and wrist strain.