light and height

got a 1000 watt MH
was wondering how high i should let the clones get before switching to 12/12
and was also wondering what distance to keep light from tops..
Also, is it necessary to put black plastic over the 4inch cubes to prevent algae?

do a hand test for the lighting
and as for how big you let your clones get, thats upto you
just remember they might triple in size.