If you don't know what your arrow length needs to be, cutting your arrow a little at a time isn't going to help. You should have matched the spine to the arrow tip weight when you purchased the arrows.

A good rule of thumb is, take one of your new arrows and draw your bow until it bottoms out on the cams or can't be drawn any further. Have someone mark your arrow about 1 inch in front of your rest with a marker when you do this (keep your draw fully back, not where you normally anchor). Most people draw their bow back to the wall, then let off a bit, get comfortable with the anchor point, then release the arrow. Typically the end of the arrow is about2 inches beyond your rest at this point, which is about perfect. You just don't want to cut your arrow too short and have it pop off or through your rest if you get excited and over draw your bow, which happens to everyone. That is why you cut your arrows to the max length when you bottom out your bow.

Don't worry so much about arrow spine when cutting your arrows, because that should have been addressed when you bought them, not when you cut them. I hope that makes sense. An inch or two of arrow makes very little difference with carbon arrows as far as loss of speed or increase in speed. You will never notice a difference. Hope that helps.Flinch