a 75 gal should have a flow between 20-40 times the tank vol in gph
In this case that would be between 1500 and 3000
Being a fowlr you could get away with between 750 and 1500 I would still work toward the high end of 1500 because you need the added filtration power that flow will give to get the nitrates and phosphates up into the water column where they can be filtered out
So Blktang's suggestion is right on the money
not necessarily anymore esp with Koralia pumps. These provide a turbulant flow rather then a straight or lamiar flow. This covers a wider area but with less directed power.
In my 30g I have a flow rate of 800 gph provided by 1 Koralia 3 and in the 110 gal I have a flow rate of 5500 gph---no whirlpools accept on the surface and that is good
The flow from the canister should count. The flow rating for the skimmer pump isn't useful since the skimmer tends to pump mostly air. If the actual water flow through it is 100 gph, that puts you at 440.
I would target at least 800 gph through the tank, although more might help the live rock with filtration.
My 90G has one powerhead sitting about mid-height pointing across the tank and one about 30-40% down the tank angled slightly up (it's a Koralia 4, so it gives me good surface break without giving up flow across as well). I'm about to add some more water movement, because it's a lot lower than you think. 1500GPH in a 75G tank sounds like mega overkill, but I barely feel my tank's 2200GPH and it's only a little bigger than yours.