I have some recent pictures I took of tangs in Hawaii. They are unbelievably fat, so thick it just looks silly. I've never yet seen a picture of a fish in a tank that looks as healthily plump as most all fish in the ocean do.
I wouldn't worry at all about over feeding your tang. Feed it as much as you can without compromising water quality. After all, they graze ALL DAY LONG in the wild, they dont just get one or two meals and then nothing. So I'd just feed to your hearts content, I've started adopting that method with my current tank, I feed many small meals throughout the day and I think, although it's just a feeling and not quantifiable evidence, that my fish are healthier and less aggressive because of it.
Long story short, I dont feel there is such a thing as too much nori. I feed lots of it, along with NLS pellets and my fish are still no where near their wild counterparts as far as weight or thickness is concerned.