I shoot a Switchback with a 28" draw and 70 lbs. I have my ACC's cut to 28 inches and I shoot 3-60's. They will only spine with a 100 gr tip. If I want to shoot 125 gr tips I have to go to 3-71's. I spent alot of money on arrows following spine charts. In other words they are not all that accurate. Generally speaking if you are shooting 68 lbs and above with 28-29 inch arrows and draw 28-30 you need a 340 spine with a 100 gr tip.