Before going too far with this, remember that you are not shooting competiton-grade projectiles. If you were shooting match-grade bullets I'd say use the Stoney Pointe gauge and a bullet comparator......but you do not have enough consistency with these bullets (I don't think) to get real value out of this excercise. I shoot various Hornady spire points in handloads and see a fair variation among them when comparing OAL's. I use Hornady dies, mostly, and mark the bullet-seating adjustment knob for its relation to a given, constant point on the top of the press. Within 1/4 turn of that knob in either direction I hope to find a sweet spot with any bullet that has a cannelure....if my rifle likes that bullet!
I'd stick with the Hornady xtp's, but vary charge weights by up to maybe a total range of 10%, where allowed, to see how accuracy varies. I'd crimp all bullets firmly so that consistent ignition is assured.
I'd vary my primers by shooting both Win Small Rifle and Rem' 7 1/2 BR small rifle primers. But, when changing primers I'd go back to my starting charge weights once again. Only change one variable at a time: brass, bullets, powder, primer, seating depth.
Pray for help with your project, as well. The .357 mag' is a good deer caliber out of a rifle! You should have fun with it.