I don't believe in a set flow rate turnover. In a fowlr most people have the fish that tend to create the biggest messes. I think you should focus on no dead spots thus keeping all the detritus in suspension until the skimmer gets ahold of it. Depending on tank and aquascaping that could be 10x or may need to be 20x, as far as flow through sump, I personally would shoot for 10x display volume if you will have big messy eaters, it will get more into the skimmer. That will also depend on your skimmer though. I run a large Beckett that can easily handle 2000gph flow through it, so I can run any speed through the sump I desire or feel the need to. What size tank and proposed stocking, and I am sure you could get more specific answers on this board.