It also depends on what you have in your tank. Some things are like kids --- they want rough and tumble, like a surf washing back and forth, while other things are like old farts - - they want slow and easy with no change of direction.
General rule of thumb is that you add up the actual GPH of everything you have that moves water. If it's less than 280 GPH (10 x 28), go clean your dentures. If it's more than 560 GPH (20 x 28), you're probably a surfer-dude yourself.
Actual GPH of your sump or closed loop pump depends on the number of twists and turns in your pipes, plus how far up the water has to be pumped.