Your calculations are out to lunch. Max flow and max head will never occur at the same time.
The formula you want to use is Watts = m(H2O)x lpm/6
Look at a pumps P/Q graph and find flow and head from there, then plug them into the formula.
As you do that with multiple flow points, you'll find the plotted points look like an upsidedown "U" with the pump being most efficient at one point. Usually that point is about halfway down the P/Q graph.
Even at the pumps point of best efficiency, centrifugal pumps tend to be horribly inefficient.
Look here for examples.