Yes, I have simplified the formula the original is
n=lQH/3960P US or n=lQh/270P metric.

Where
n = efficiency coefficient,
l (lambda) = specific gravity of the material being pumped for water this
is 1,
Q = the volume in gallons/minute or M3/h
H = the head or pressure in ft or meters
P = the power in BHP, 1 BHP = 746W (if I remember correctly).

As the formula has no exponents in it, I believe that it is linear.
Having taken one of my pond pumps apart I would guess that the value for
n is very low, probably not much above 0.1 and maybe even lower.
AXO