position of 400 watt

i got a 400 watt mh and hps my grow box is 5 ft tall my plants are about four inchs how far from the plants do i postion the light above them ? The exact dimensions of box are 5 ft tall x 4 ft wide x 3 ft deep

I would place the light roughly 22" above canopy to start, then lower light every other day until its perfect.

thanks for the tip man!