However many times you would like it to be turned over
There is no need for tremendous flow for a FOWLR, so you can go on the light side and then add flow as you think necessary. Maybe a couple of Seio 1100s for a tank that size would be fine flow -- but less or more would be fine too. It's your call. Get a powerhead and see what you think.
For the sump, you should aim for 3-5x the display volume for flow through the sump. So for you somewhere around 500-600 gph. You can add flow to the display with powerheads or a closed loop.
Unless you have some incredible overflow setup, you can't go 10-20x through it. A 1" drain in a 90g will only yield 600gph. I learned the hard way... someone recommended the 10-20 and sold me a huge pump that I really didn't need. I believe the recommendation and probably what you're looking for is the MAIN pump exchange rate and that should be 5x. IMO, the rest should be made up in Maxi-Jets (or equivalent) inside the tank and don't forget the Protein Skimmer pump is an exchange equivalent as well.
There is a difference between display turn over and flow through the sump. Generally people don't aim to achieve all of their flow through a return pump from the sump. For most sump setups that means fighting gravity which means a bigger pump which means more heat and higher electric bills. Shoot for 3-5x display for flow through sump, and then get the rest of your flow through power heads or a closed loop...
I like to have a fair amount of flow in all my tanks, whether FOWLR or reef, to avoid any dead spots. I'd aim for 20x turnover; with higher flow the nitrifying bacteria will be able to do a better job and less detritus will collect in the rockwork.
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.
Sounds right to me, if it's the only pump and it's pumping at full capacity, but you do have to take into account things like head loss (if it's pumping up from a sump, the rate is reduced by a certain amount per foot of height the water has to travel).