Programming question! :)

[Jose]

I have an idea. Depending on how you want to optimize it (apparently for speed rather than size) you could organize ranks comparisons in a binary tree like sequence (assuming the ranks are fixed). This would grant you the fastest search results possible but probably a tid bit bigger code.
Don't ask me for a code sample right now though

[Jose]

Is the size between boundaries constant ? (i.e. is the total nr. of ranks a multiple of the size of one rank).

[Jose]

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)).