What Mule Deer said.
Quote
One of the best things to do is try a bunch of rifles with different lengths...

I've done a lot of fiddling with different lengths of pull on different firearms and I get the best fit a little short of average. Don't know why, I should be average or just a little longer by the standard advice. Not that it makes a great deal of difference with rifles though the scope aligns automatically with my eye when the fit is right.

The key elements in human thinking are not numbers but labels of fuzzy sets. -- L. Zadeh

Which explains a lot.