<a href=showthread.php?s=&postid=10467007#post10467007 target=_blank>Originally posted</a> by der_wille_zur_macht
Again, you don't want to measure flow on the return. Even though the return pump/plumbing IS what determines flow, as you've stated, it's also the case that altering it at all will ALTER the flow produced, so you won't really be measuring the flow in your system in an unaltered state.
Even trying to measure by holding a container at water level wouldn't be effective, since the bends and turns in the plumbing can cause as much head loss as the actual vertical distance pumped, if not more. So unless you were holding the container inside your tank to catch the water as it came out of the unaltered return plumbing, you'd be altering the measurement. And of course holding a container around the unaltered return opening is not possible when the tank is running.
However, the drain line running to the sump is just a gravity fed mess of air and water, with the volume of water always being 100% equal to what the return pump is pumping. Hence, you can measure the flow rate there without effect on the result.