The easiest way I think would be to turn off your return and let all the water that is going to drain down into your sump drain.
Once it has stopped draining, then add some top off water to your sump to just below the highest level you want you water at in case the power goes out.
After that, turn your pump back on and that should bring the water level down in your pump chamber.
You can place a mark on the glass so that you know where your 'full' level is at.