Controlling it is pretty easy - you set the water level you want when you initially set up the sump, and then you maintain that same level when you do water changes and top-off.
To figure out how much water in your sump has a couple of factors. the most important one is to make sure you leave enough empty room in your sump so that when you turn off your return pump, there's plenty of room for some drainback from your display tank to be held in the sump. For instance, if your sump is below your display tank, when you turn off the pump, all of the water that's in the lines between the return pump and the outlet, as well as some of the display tank water, is going to drain back down into your sump. Make sure you have enough empty room in the sump to hold all of this.
Another factor is if you're running an in-sump skimmer. Most skimmers have a nominal operating depth, so you'll want to make sure you've set it up so that your skimmer operates properly.