I'm only going to get you a little closer (I've been attempting the calculations for my system too) Maybe at least a bunch to think about.

All those fittings, including the coupling and horizontal lengths of pipe add a lot of head loss due to friction. I started to do some math for you and got all sidetracked. I'm not sure how much head you add because it's based on the flow rate, the higher the flow the more those elbows, etc slow it down. You could easily have 6 more feet of head than the vertical difference. It's not that much because you have flow. But each of those slow you down more. But as the flow slows down, the friction of the fittings affects it less.

You can find a chart on the web that will show you the head loss due to friction per 100' (or look in a pocket ref)

I don't know if this makes sense, but I'm thinking about sitting down and trying to come up with some stuff that will be meaningful to us piping tanks like you and I are doing.

You could always rig something up to measure how much water you're pumping by measure water level changes. Lower the water level in the tank (below the overflow, gotta take that out of the picture), fill the sump as much as you can, calculate the water volume per inch in your tank. Start the pump and determine how much volume reached the tank in a certain time period.