I'mback
Comfortably numb!
That is why the only measurement I respect comes out of a PAR meter. Distance IMHO is nothing more than a shot in the dark. OTOH with the proper tool, you KNOW exactly what is going on.Keep it at 60 cm (~23"). You'll fry them at 40.
I simply cannot wrap my head around folks that seriously grow not own one? The F..... pays for itself 1st run!?