Simple answer would be as low as the plants will tolerate, if the tops bleach a little lift em up higher and the bleaching should recover. 4 inches to 3 feet in practice. Led's have near zero Infra red so they don't burn the plants they just bombard the chloroplasts till they turn white from simple overload of photons, this is rarely irreversible so not so verboten as hanging hps too low. If its not too late and you have been tricked by the blurple marketing wank, cancel the order before its mailed, buy white spectrum cob type leds instead, they will cost far more watt for watt but you will either abandon leds and go back to hps or buy white cob leds, the growing efficacy is far superior to anything hps or blurples can achieve imho.
) foot to a foot and a half from the canopy.