The rate at which water enters the sump by siphon or direct overflow should have a range depending on how high the water in the tank is. For example, if you dump a gallon of water in your tank, raising the level, more will overflow, and if the level drops for some reason, the overflow will slow. Ok, so the level of the water is determined by how fast the pump returns it. As long as your return pump is powerful enough to pump water back to the tank faster than the minimum sustainable siphon/overflow you can achieve, you should be good because even if the pump is more powerful than the minimum overflow rate, the extra water in the tank will cause the overflow to increase, compensating. That's the basic principle. A specific system (especially a homemade one) will require some optimization. An adjustable flow pump helps too... a lot.
