There are two programs on the market right now that work very well. TAP (The Archery Program) and On Target from Pinwheel Archery. I have On Target 2 and have used TAP before. Both work pretty well but take some playing with to get used to them.

They have data bases with different arrows and thier spines and most bows on the market with the effeciency rating already programmed in. Or you can custom set all the parameters if you want. I find they are most accurate if you shoot a few arrows thru a chrono and use that as a referance. If you know the speed of a given arrow weight you can set the program to be pretty accurate.

It is also handy if you want to shoot an arrow length other than what is AMO standard for you bow. You just put in what your draw length and draw weight is and what length arrow you want to shoot and it will either list what arrows will work or you can pick a certain arrow and it will tell you if it's good, or how far off it is. With the online calculators and arrow charts they base your draw length off from your arrow length, so it needs to be the correct length.

The programs do many other things as well.

I can't believe that most archery shops don't use this sort of software, boggles my mind sometimes. I mean that's fine if you have been in it for a long time and know what spines will work with what bows, but this isn't the case in most shops I have been to.

There probably is a formula, but I couldn't tell you what it is. There are a lot of things that effect what spine arrow you need in a compound bow.

I think most of the charts and online caclulators are based on a hard or speed cam being around 300 fps IBO. So if you are shooting a bow that has a faster rating than that you might want to pick a stiffer arrow than what the chart says you should have.

Paul