If you are talking about just dampening resonances, there are a lot of 1/2" and 3/4" foams that you can use on the insides of the speakers box. Parts Express and others sell it and many speaker kits come with it.
If you are talking about poly-fill and tuning, then it's a trial and error setup. I've seen many boxes that are about 75% filled with the stuff, but the end result is tuning the effective box size to achieve the proper bass balance. You are affecting the hump and roll-off by stuffing, as more stuffing makes the box "look" bigger to the waves inside. Obviously there is a theoretical end to this. Full stuffing is (IIRC) 25% more effective volume than the box alone.
But calculate all you want, the true test is to stuff, measure, restuff, remeasure, repeat.
Check out "damping" and "polyfill" at Parts Express -- a lot of good info, reviews, and recommendations there.