Modern designed compounds are so efficient there is very little variation in results of various spine arrows within a reasonable window.
If anything, the stiffer the better. And it is nearly impossible to be "too stiff" to shoot accurately.
But only the very under-spined shafts will show a measurable difference inside 20 yards.
With field points.
Broadheads and all bets are off.
Being excessively light in gpi is dangerous regardless.
OP, in regard to your question on vane positioning make sure all vanes on the individual arrow are the same. Some variation may be seen from arrow to arrow with vanes further back, or further forward but the key thing is for them to be balanced and even on each arrow.
FOC is something to chase for momentum calculations and those seeking deeper penetration, especially when shooting lower poundage.
Just set up the arrows the way you want, following good charts on poundage/length/spine and try to build every one of them exactly the same.
Consistency.
Hope you are improving and on the mend!
