Ok.
Here are the common Eheim curves.
In red, I drew the line I was talking about, along with the perpendicular. Where that line intersects the curve, is where the pump is most efficient, because that's where it provides the most amount of flow for the pressure.
In green is where you measure it all.
For a 1046, it is most efficient at 3 lpm, with 0.75m head.
For a 1048, it is most efficient at 6 lpm, with 1m head.
For a 1250, it is most efficient at 11.5 lpm, with 1.3m head.
Now if you're choosing a pump, you do this exercise in reverse: figure out the flow/pressure drop that you need/want, and select a pump with the above info, so that it's most efficient at it.
Note that most people achieve 50 gph (~4 lpm) in their rigs, so I can tell you with confidence that anyone using an Eheim 1250 isn't running it efficiently.
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