Turn the power off. Let everything drain down. Once it's all settled, you can fill the sump as far up as you like. You'll never get more water in there than that. Once you turn the power back on, the return section will reduce down, and that's the height you need to mark so you know where to top off with RODI as water evaporates. If you have an ATU, you set the float or sensor to go off when the level drops from that marked line. Does that make sense?