"Look at a pumps P/Q graph and find flow and head from there"
Yes, just need to specify that the flow and head are found at the intersection between the pump P-Q and the system P-Q curves. The other points of the pump P-Q are pretty much irrelevent here.
Hence you need to know the system P-Q, if you want to select the best pump.
Using max flow and max head corresponds to the pumps' useful output at full flow and (ironically) zero head. (It equates to the kinetic energy of the water) So it does actually have some mathematical basis, to give credit where credit is due
