If the intake and the outflow nozzle are 3 feet above the pump, the flow will be 250 gallons per hour. If it is a canister filter, that figure is for the media baskets being empty. If it is a submersible pump, it should be somewhat accurate. Of course, length, diameter, and curvature of hoses will have an effect.
You need to find a sump guru who can tell you what kind of flow is good for your set up. I know HOB
overflows have a maximum flow rating, so that you would not want to exceed that rating for your return pump. But built in overflows should be able to handle quite a bit, and 250 gph
does not seem excessive for a 90 gal
tank. In fact, you probably could run two of them?