i would say start to flower at under a foot aim for 4 ounces wet witch isn't hard for a small plant. but why would you wanna limit the plant to one ounce? with that light you could get 1 pound per plant with 4 600w watters no?
with 400 watts i'd flower before 12". after the stretch in flower, you're gonna end up with a lot of plant. the 400 watter will only penetrate so far into the canopy.