325-350 grains. 5 grains of arrow weight for every pound of draw weight. Arrow length has nothing to do with grains. Length is for figuring spine.How light of an arrow with field points can i shoot with a 28 inch draw shooting any where from 65-70 pounds .... right now im shooting 400 grain arrows with 100 grain field points and wanna get something lighter to gain a little bit more FPS
65# x 5 grains = 325 grains
70# x 5 grains = 350 grains