I second bens on his #3 point.
Doing float exponents take a lot of processing power, hence a lot of time on a tiny microcontroller.
You might just want to do:
y = 359.56x
and see if the error really matters or not. So I calculated error for you:
(359.56*x^(-0.8702))/(359.56*x) , where x is between 0 and 255 (ADC reading)
x=2, error is 27%
x=5, error is 4.9%
x=10, error is 1.3%
x=50, error is .07%
So in most cases, your error will be under 1%, which just isn't worth doing floats and exponents . . .