Thanks for all the info it's been a lot of help! Did yours come with grey PVC for plumbing? If it did was it schedule 80 PVC?Yes the overflow will stop draining to the sump once the water breaks below the teeth. That being said I used loc-line for my return and it created a siphon which will draw water back until the loc-line is exposed. That was too much water for my sump so I added a GF Wye valve in the return line. this worked but I had the valve fail several times during feeding after about 6 months. I always caught it in time but it un-nerved me that I could loose power or the return pump would fail and too much water would back siphon out. I fixed this by drilling a hole in the loc-line on each run just below the surface of the water. This breaks the siphon almost immediately.
I'm not sure exactly how much water drains back, for me I would say its less than half a gallon, whatever is in the drain and the return already that drops back down in. I'm really excited to see your build, please ask away if you have any other questions or want to see detailed pics of anything that I've done with my setup.