Really not that hard and no need for a formula. You just need to know the
layout of your
dimmer rack (what channels are on which of the three
phase legs.) The object is to end up with equal wattage loads on each of the legs. Close is good. Closer is better.
For example, lets say you had 24k of lights being used on a show (make it simple, 24 1k lights) and channels 1,4,7,10 were on one
leg, 2,5,8,11 were on the second, and 3,6,9,12 on the third. Putting 2 lights on each
channel would give you a balanced load.
If you had 6 k to balance and your
layout was 1,2,3,4 / 5,6,7,8 / 9,10,11,12 then you would put 2k in the first group, skip a couple of channels, put 2k in the second group, skip a couple, and 2k in the last group.