How to get flow right..

[jitgobbler]

How do I get flow to be right, I can’t tell if high or low amounts are good because i see frag tanks with essentially no flow and corals look great and then some reef tanks have super high movement and some have very little too…I feel like in mine I try both and still struggle with my euphylia and other corals what do I do to get it right??? I have two wave makers on sine mode pointed towards front glass diagonal shooting from both back corners in my 20 gallon.