I’m in almost exactly your position: trying to figure out the right X10s for a 43# (OTF) w/29.5” (excl. point).
I know the Easton chart says 500 spine or so. But I have ACE test arrows of all spines 520-720 in sets of four (two fletched, two unfletched). I fitted these with 120grn points to the 570s, 620s and 670s and tried them at 40y. The answer was 620s and 670s were closest grouped, 570s were way apart. So the best ACE spine looks like 650.
This was not a surprise, as I’d had a dialogue with Timid Toad elsewhere on this forum which concluded that practical tests are a lot better than spine charts - and because my other bow, at 39# OTF, is best tuned with 670s w/100grn points - which is miles off the Easton chart reccomendation for 39# (in fact I can even tune-in the 43# bow to group the ACE 670/100grn tight at 40y but the button has to be very hard, about 8N. In other words, it’s possible to use 670/100grn arrows all the way from 36# to 43# just by increasing button pressure from close to min to almost max - but that’s another story).
So... though the Easton chart says X10 500, I’m pretty sure the right answer is X10 650s (or maybe 600 at a push, given that 650s come in 29” max).