I've written about it a while ago in another thread - I believe it was about copper compounding, but I wanted to write it more fully.
The idea is to think of the parameters of the stored attribute in a metalmind and write an equation based on that. What I have now is:
I = t*a*p
With I as the stored investiture, t as the time you took to fill the metalmind and a as the amount of the stored attribute. While tapping, you can press more of the attribute to less time, and it stays the same amount though it lasts for less time than it should, and so p is the "resistance" when pressing the attribute. While storing it's set as 1, and so, apparently, is time in copper (and maybe nicrosil).
Any thoughts?