That is true -I have a bunch of gauges -but can't think of many with a low enough scale. I should be ably to find one that will go low enough to verify the pressure head and gpm -through my flow meter- at certain water column heights. If I find a few -I will send you some. I doubt I have any new ones, though...But I will look tomorrow.
It will be easy to time a measured volume through the pump also -to compare to the flow meters to gauge their restriction. Mine has two additional 90 Deg. bends than yours:
http://www.kinginstrumentco.com/7520...7520_7530.html
I bought it off ebay -cheap- with thoughts of including it in my system.