I agree with jmaneyapanda - it's pretty much impossible to overfeed a tang, but it is important to feed the right things.
I offer my yellow tang (and her tankmates) nori, dulce and wakame seaweeds on a clip - at least one of these is available to them daily. They also get a rotation of 'blender mush' (homemade blend of seafoods and seaweeds), flakes soaked in Selcon, pellets and all the live-rock grazing they can handle
The tang is enormously fat, as are the rest of our fish, but they're far from overfed. To me they look as they should - not the typical scrawny specimens we see at the LFS, which have been starved for up to 2 weeks prior to and during shipping.