When to put seeds under MH???

My short ryder autoflowering seeds just sprouted yesterday they are under a 2bulb floro ( not t5 ) in red party cups... When will it be safe to put them under a 400MH? keep in mind this strain can start sexing as early as 3 weeks from sprout. + REP for info

Just IMO, but I think the norm is to wait till they are 3 to 6 inches, I could be wrong but it wont hurt them.