You just need to factor that equation to single out inputForA. Here are the algebraic steps:
output = sqrt(inputForA / range) * inputForA
output / inputForA = sqrt(inputForA / range)
sq(output) / sq(inputForA) = inputForA / range
range * sq(output) / sq(inputForA) = inputForA
range * sq(output) = cube(inputForA)
So the cube root of range times the output squared should give you your original input. I don't have a good way of showing that in here though...