Couple of things.
I've got a Dart and had the impression they are a hi volume but low head pump. Good for CL, but not return, might want to recheck the curves.
http://www.mdminc.com/Dart.htm 50g isn't that big of a sump, specially considering it won't be filled to the top, sure you would want that much flow anyway through it?
For the CL intake could have it lower to help move the water a bit more. Also the flow seems very low.
If the CL intake is below the water line (for it to be a CL) make sure you don't have any vertical head in the calculator.
Pump curves:
http://www.reefs.org/library/pumps/