From the look of things, your holes are in the back? If I were you and that is where the holes are positions, I would reverse what you have. The overflow to the sump should be higher then the return line. On the return line I would also put in a check valve (one way water flow switch) in the event of a power outage your not going to have all your water come as low as the return. As for pumping you need to figure out the gallonage of your tank then times it by 10 and find a pump pumping that amount. Alot will say you can do less but with head pressure and such its safer to just go with a straight 10x. I hope this helps any questions you can pm me directly.