Sometimes it seems like a voodoo science. There are a lot of factors that can play into it, particularly the species housed in the tank, but also things like aquascaping, substrate, feeding habits, husbandry routine, etc. General rules of thumb are a bit dangerous, but as a benchmark softies would be more in the 10-20 range and stony corals
more in the 20-50 range. That's not to say that a stony tank couldnt thrive in a low flow setup (or softies in a higher flow setup), again it depends on the specifics.
If you'd like more direct advice, could you post some pictures of the current setup and describe the inhabitants that you do (or would like to) keep?