I agree with all you said RC!
Maybe i was looking at the drawing and seeing the refugium tank being shorter then the sump tank. If there equal size tanks and the water line at shut off stops at the smaller tanks hight it should work as you say. If the refugium is much smaller like in the drawing then the water in the sump will force over to the refugium causing an overspill. Am i making sense?
I didn't see your post as being a "smartie pants"
It was a good post imo!