The old adage for string length is: - 4" for recurve & - 3" for longbow depending on length of bow from string grove to string grove. That was established from years of use with Dacron strings. What formula would be good you think using the new string materials that don't stretch near as much? I made a string a tad short with TS+ the other day that I wouldn't have worried about stretching enough if I'd used B55, all I've ever used up until now, a Dacron material.

Actually string grooves. Whoops!

The "old adage" only works on some bows.  AMO has always been 3" shorter (under tension), but that only works on some bows too.

The best solution I've found is measure the string that fits and make the new string accordingly.

How much stretch/creep you get from a string will depend on the material, how many strands, draw weight, temperature, how it's made, etc.

Sorry, there's no "one size fits all" answer.

Chad

It is a trial and error thing every time I try a new material. When I get it figured out I write it on the back of my jig for future use.

exactly what chad said