In my opinion the least amount of Light I'd put over the 75 gallon would be a 4x96watt power compact setup, due to the low depth of the tank, the power compacts in this case would provide excellent results with most corals, soft corals, LPS and even some milder sps corals like pink birdsnest.. A 4x96 PC setup will also be friendly on your wallet, a retrofit kit is even cheaper I believe if you have a canopy over the tank
That would give you 5 watts per gallon which is pretty descent for a good variety of corals...