There are two types of magic stats--the one shown on a ring (1~10). And the one you can increase by gaining xp and equipping better gems. Have you ever been confused by them and their relationship to the ultimate damage/health regen you could do? Well, I was. That's why I set aside some time to experiment the difference between the two types of stats. I battled the same enemy over a hundred times with different rings and magic stats (1~450), and created some plots with an online software:
Long story short:
1. The magic stat on a ring is similar to a linear multiplier, but not quite. If x axis represents the magic stat and y axis the ultimate damage you could do, then the plot of a ring magic stat is in between a straight line and this. But it looks more like a straight line.
2. The magic stat we get from xp and gems is a standard diminishing returns function. If x represents the magic stat and y the dmg/health regeneration, then the plot looks like this
Darthduck also had an awesome post on ring stat and their regen time here. Check it out.