I see this equation more useful for getting the flow information which is generally not available while head & power can be measured.
The formula is useful to make decisions for improvements in pumping system.
The equation is
Eff = 80 - 0.2855 * F + 0.000378 * F * G - 0.000000238 * F * G^2 + 0.000539 * F^2 - 0.0000000639 * (F^2) * G + 0.0000000004 * (F^2) * (G^2)
Where F = Developed head in Feet - F should be 50 to 300 feet
G = Flow in GPM - G should be 100 to 1000 GPM
So if you know flow & head, you can calculate approx efficiency & then can confirm this with power consumption. OR if you dont know flow then use iteration method to calculate flow based on actual power consumption.
Thus it is very useful for all process engineers who need to improve their pumping system. This formula can definitely guide them in making proper decision on replacing or repairing the pumps.