I'm not sure if I could give you a formula, but going on some level 50 stuff posted by Nybling at Tamriel Foundry it would seem with Crystal Shard the sorcerer ability that every 800!!!!! points into Magika would raise the damage 100 pts. Once it reaches soft cap, it goes up by 50 points per 800. Soft cap is currently set at 1900 for both magika and stamina. Also playing around with the Esohead skill bars (even though there is a lot of errors), they all seem to follow this trend. Spell power however seems to raise damage the most for just 10-20 pts. Now Esohead could be all wrong, and Crystal Shards could just scale real bad, but I'm guessing that there is a trend to this since all of ESOHEAD follows this, but again i could be wrong.