Maths Question

I’m currently using flash with a teleo board (hooks flash up to little electronic inputs) and im trying to use light dependant resistors to change the alpha of a mc.

The light dependant resistors give flash a reading between 0 and 32. 32 being bright light, 0 being complete darkness. Now I want to use those numbers to make an object 100% opaque in complete darkness (reading 0), and 0% in complete lightness (reading 32) then all percentages inbetween

My code so far

mc_a0.onEnterFrame = function()
//ai0 is var setup for each LDR
var v:Number = ai0.getValue();
mc_a0._alpha = ((v / 32) * 100);

Now that code works. but it does it the wrong way round. everything is 100% on reading 32. and everything is 0% on 0. I need reading 32 to = 0% and reading 0 to = 100%…im tired and can’t see what to do to swap the figures around.

thanks in advance