A quick, easy way to roughly test spine is to take an arrow that flies well from your bow. Drive two nails in the wall a couple inches less apart than your arrow is long. Put the good arrow on them, hang a weight from it, and mark on the wall how far it flexes. Take your other shafts, hang the same weight from them, and compare how far they flex to the good one. It'll get you in the ballpark at least. After awhile, you can judge them pretty good by hand flexing. 5/16" is a bit small for hunting weight bows unless it's some kind of really heavy hardwood. I have made 5/16" sourwood arrows that spined at 55/60#, for example. It also has a lot to do with draw length. If you have a shorter draw length, (like my 26",) you can often get by with smaller diameter shafts.