Gloriously simple equation for that, so long as your two pivots are rigid enough haha..
My jig was made primarily as a string jig, stretching is just as easily done with a hook and a weight so.. Personally I just stick it on the bow and put a few dozen down range using nice heavy arrows (larger inertia) to stretch it in before serving. Or even just string it for a few hours, realistically its always going to stretch after arrows, whatever you do.
Still, stretch with fastflight its well, well below 1% so over a 150mm stretching area its around 1mm.
I've, when rushed, made and shot competitions with strings that I threw together minutes before and honestly it made no difference. People get obsessed about things that make fractional difference compared to their own variation shot to shot, no matter how good you are.