Here are the Eheim curves (below).
Once setup, all you have to do is turn it on, and measure the height difference in the two columns of water. BTW, if your tubing run is too short, you'll have water on the floor.
Tke that measurement, and find it on the graph. draw a perpendicular line, from where your reading hits the 1250's curve, and you'll know approximately, what your actual flow rate is. (unfortunately, everything is metric, so a little math might be in order
See
www.convert-me.com )