I'd just drop the formula altogether, I think. If at the end of the day, the numbers are just worked out so that you and the employee agrees the salary is about right ("I want $x, so I guess I'd better get a 1.3x experience multiplier"), then just make "work out the salary such that you and the employee agree it's about right" the policy. The transparency keeps these negotiations simple and fair; for example, pushier people won't get more raises than their shyer colleagues because everyone will notice and act accordingly (including you, who will find it easier not to give pushy people those raises when they're posted on the company website).
