Another idea that I don't know if it would work. If the values of the curve can be guessed by using a mathematical function you could revert the function and get the x value. Then calculate the rank by dividing by rank x size (of course that assumes rank x size is constant between ranks, as opposed to rank y size which isn't (like you say it's a curve)).
On an 8bit microcontroller, you really want to avoid multiplication and division (unless you can use a power of 2... ie left or right shifts). I really did try to do this mathematically first, but realised that I simply don't have the CPU time
