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:

error:

(359.56*x^(-0.8702))/(359.56*x) , where x is between 0 and 255 (ADC reading)

Plotting it:

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 . . .