You set your water level by adjusting the height of the overflow box teeth. This gives you your minimum water level. The actual level will vary a bit with the pump flow .... since there will be a layer of water that is above the teeth.
In practice, the overflow teeth are designed so that the layer is minimal (1/4" or so). The math is a bit messy. Essentially, the layer of water has to be thick enough to exert enough hydrostatic pressure to overcome the resistance to flow ... it is much easier to raise or lower the teeth with your pump running to get your water level, rather than trying to calculate it.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.