I shoot a 65@26 centaur and shoot 29" arrows. Axis 300 shaft with a 100 grain insert and 200 grain tip flies perfect. Problem is I only have one. I have about 20 340 shafts at 29". How much would you guess I would have to take off a 340 to make it the same stiffness as a 300 with the same weight insert? A half inch? One inch? Just getting guesses before I start cutting shafts. I have a couple inches to play with. Thanks

No clue, but I do know carbons respond quicker to length than they do tip weight so you're on the right track.  If you need a little more help, you can add an aluminum footing on the outside.

Take one and cut  a 1/4 inch off the back, reinsert the nock and shoot it. Continue this process until they are flying good or you run out of arrow and it is coming back into the riser. I wouldn't shoot two different length of arrows though, the site picture will look different to you if you have a consistent draw.
The trajectory will be hard to learn if your shooting instinctive. I bet it is at least an inch and you will run out of arrow in my experience.

27.75 to 28" should do the trick for you.
I used Stu Miller's Dynamic Spine calculator as found on ThreeRivers.

https://www.3riversarchery.com/SpineCalculator.asp?pass=2

Go Stu Miller! That program has helped a bunch of folks get into the right zone. Roy

As I recall my Centaur was cut past center. That fact makes that bow very forgiving with arrow spines. I think the bow will shoot them just fine as they are. But you can shorten just one a 1/2" at a time to find out how short you can go if you haven't epoxied in the inserts. If you have you can cut them from the back end to shorten also.

I would shoot one and see for myself before I start cutting them.

I wouldn't cut them more than 1/4" at a time.