Suggestion is this: Sp regen, without items boosting +magic, should relate to Max Sp. I don't think this should be a BIG factor, as items can give +magic, but all the same, I think that the higher your Sp., the faster your SP needs to regen.
That makes sense, a more powerful wizard would regen faster (because he would need to to cast his more powerful spells) -- but not fast enough that a lvl 15 spell is as easily cast as a lvl 1