well as a simple answer, as close as possible.
I had someone tell me the red's are what bleach plants, whites you can put as close as possible ( without burning from heat ).
how many watts is the panel? lens' on the led? I would estimate you can go as close as possible, provided you fill the space you need first, and don't cut corner / edge light out..
I've had a 270 panel with 60 degree lens 8 inches from plants ( as that filled my area ) , but the lumigrow, anything under 12 inches "bleached" the tops, which might make sense with the 10 watt red chips.. also had cree xt-e red that bleached within 3-4 inches I believe, can't fully remember that one, but they were damn close.. and with lens