I think the equation you are looking for is called head pressure. You need to put together a plan for your plumbing then look for a pump that will give you the GPH you are looking for AFTER deducting a few things. You know you have to lift the water 45", so that is almost 4' of head pressure. Then add head pressure for any barbed fittings, gate valves, check valves, 90* elbows, 45*elbows.
I
think it goes as follows;
45* elbow - .5'
90* elbow - 1'
gate valve - .5'
ball valve - .5'
barbed fitting - 1'
10' of horizontal pipe - 1'
1' of vertical pipe - 1'
Find out how to calculate head pressure and properly size your return for a saltwater reef aquarium.
www.marinedepot.com
Don't worry about the 3/4 pipe. That can push up to 1,380 GPH, and drain up to 780GPH per
http://calctool.org/CALC/eng/civil/hazen-williams_g
Of course, this is in perfect conditions so your results will be different.
Pistondog has given you good advice though. I'd go with a DC return pump. That way you can easily tune it to EXACTLY what you need without overwhelming your overflow.
Clear as mud! Hope I helped and didn't confuse you more.