...er I don't know mate....at a guess, I would say you are ok at 24" .....LED and CMH are very different lights...even tho our wattage is the same, the intensity is very different.
oh my apologies for some reason i was thinking you were running a dimmmable 600 w led. didnt realize it was cmh. my plant seems to be happy regardless, i can already see a growth improvement since yesterday. so think im good haha.