I like 40x the tank volume unless you are sps heavy. So for a 56, a little more than 2,200 gph. That's just a starting point tho. It's really unpredictable how the water will bounce off the rocks and glass, and the patterns from a big powerhead are really diff from a couple little ones. I don't count the return, or any hang on skimmers or reactors in that number.
You just have to play around with it. The goal is to have no dead spots where poop settles, stale water from the bottom being pushed up to refresh at the surface, and a good amount of movement at the surface so that you don't have a greasy film. The tank uses oxygen so the water must pass by the air so things don't suffocate. Also, the flow brings food to the corals but lps don't like to get whipped around too hard, they prefer a sort of "fluffing" flow over strong blasts from one direction.