If the jig isn’t marked, you will just have to experiment and measure test lengths of string. Not all jigs have the same setup so they can all be different. That is one reason I never use a jig, I just make the individual strands 20″ longer than what I want for a finished string and go from there. The furthest peg position will give you the longest string but 50″ is for about a 54″ bow so shouldn’t be the longest position for most jigs I’ve seen. You should be able to get up to string lengths for a 70 or 72″ bow in most cases.
Once you figure out the length each post position gives, mark them for future reference. Then you will have more experimenting to do by building strings to see if your string twisting technique actually gives you a string length that is what you want OR what you have marked.
Arne