Why do you want to cut it down if it flies perfectly? Too long of an arrow is seldom a problem. Too short is. I shoot a 28" arrow though my draw is only 24. Leave it alone unless you have a very good reason to cut it down 3 inches.
There are some formulas regarding spine and how tip weight and excess length affect spine but they are guestimates at best as shooting style,(cant or vertical), type of bow(centershot or not)will have a bearing also.
Getting perfect arrow flight can be difficult. If you have it now,leave it alone and be happy.
Go Noles