How many of you have read on forums that it's better to have an arrow too stiff vs. too limber? Well, I take issue with this advice. The whole idea of tuning a bow is too make adjustments so that the spine fits the bow as perfectly as humanly possible. Not stiff or limber.
Say a computer program shows that you're on the fence with a certain arrow showing maybe a touch weak. Guys almost always say to pick a stiffer spine. I say, "Why not back the bow eight down a couple pounds and get it perfect". Target, 3D, or hunting doesn't matter. If you have your bow maxed out and pick the stiffer shaft then you have no place to go except maybe playing with point weight or fletching type. The bow is infinitely more adjustable and easier to adjust than playing with shaft length or arrow components. Take the easy way out.
Case in point is the last three Martin bows I've tried to get to shoot bare shafts. At roughly 27" draw and 42# up to 50# I've used various spined shafts and couldn't get them to bare shaft well. Shafts used were GT UL 500, 400 and 22 Series (300 spine), all with 80gr tips. They always plane to the right entering a 15 yard target at nearly 45 degrees and 8" to the right. Hmmm, shaft to the right (left paper tear) says underspined, right?
Wrong!!! I don't need a computer program to tell me this. One day just for the heck of it I dug into my box and came out with a 100gr field tip which with insert totaled 112gr tip. Screwed this into a UL 500 and Lo and behold got nearly perfect bare shaft flight out to 20 yards. Well, I don't want to shoot that heavy of a point (3D setup). But at least I had figured out that a 500 shaft was too stiff (not weak). Got some UL 600 with 80 gr tips (insert + 65gr screw-in point) and flight was pretty good. Not perfect, but close. Add one turn to the limbs to add 2# and Walla. Bare shafts and fletched arrow hit the same POI out to 35 yards. Bow is now at 47#.
My point? Don't get all rigid about having to shoot the bow at a certain draw weight. Limb bolts are not decorations or just for holding the limbs on the bow. If on the fence with arrow spine don't be afraid to take the weaker shaft and just turn the bow down a couple pounds. I doubt any animal is going to know the difference between 70# and 67#.