If your first chamber is your return, second and/or 3rd is skimmer or filter/bio ball media and last chamber is return, they will be staggered. The first return you can't set, the second should be a couple of inches or more below the first and the return (last) chamber should be lower than the chamber before it. So return will be lowest chamber.
If you fill return too much they will all be higher than they should. Not a huge deal, scoop some out of the display and then shut off return pump and let the air burp. Then turn return back on and it will level of you took enough. Might have taken too much and return is low, if so just add to return chamber a little.
When topping off, always add to return chamber where your pump is.