My contraption has a trigger and sights. When I can close my eyes, shoulder my el-cheapo try stock, open my eyes and be perfectly aligned with the sights, I know the measurements I need to build a gun that fits me.
I like to hand one of my guns to someone my size that who has never shouldered a correctly fitting gun, I tell them to close their eyes, shoulder the gun and tell me what they are seeing when they open their eyes. They always say "WOW, the sights are in perfect alignment".
Anyway, my try stocks fancy trigger.