yo puff... well IME natural yellowing starts to occur first on the lower leaves. but if your pumping them that much nutrition no yellowing should take place until flush. except lower leaves receiving less light

but like you said the hps throws the color off a lot, so I could be completely wrong, but i'd be concerned about yellowing tips..

the best bet would be to browse some nute burn pics and find out

i'll be here for the rest of the grow though :jointsmile: