I mean, how do you think the manufacturers came up with the pump curves? Do you think they assigned a PhD M.E. to derive an equation using the geometries and CFD?
No. They hooked the thing up on a test stand, ran it and said, "Ok, someone write this down. At 3500 RPM, this pump is delivering X GPM at Y differential head and is drawing Z brake horsepower."
So, if you have Z HP, it makes perfect sense that you can work backwords, as long as you follow proper procedure working from the water reference point defined by the curve, consider reasons for possible curve inaccuracy (wear, etc.) and double check using a second point (differential head for example).
I swear we have a tendency to over analyze everything. But what fun would an engineering forum be if we didn't. I apologize for being cranky, laying people off is no fun and brings about a negative attitude.