Agreed. Just measured it, was at 22". I raised it to 26". From what I understand, this is about a far away as you'd want to take it.How far is the light from them? Could be stressed from a 600w blasting them this early.
I have had light stress before and its kind of that even yellowing like that. Its a delayed effect and I thought it was nitrogen and stuff like that too until it was really obvious a week or two later. Just another thought.