Well you learn something new every day. Reef Rig it is
I prefer to have a refugium as well.
When I was trying to determine how full to have my sump/fuge I like to fill it so that the water is about 1/2 to 2/3 of the volume of the sump with everything running. Then I turn everything off - simulating a power failure to make sure that there is enough room for the back siphon of water without overflowing.