Excel 2007 : Conditional formulas

Quick background information:
I'm trying to record some information about my boyfriend's video gaming, specifically how far he is in certain achievements. One column is how many points he has towards something, then the next couple columns are the different ranks of the achievement and how many it takes to meet the rank. ((See workbook.))

I have inserted a column that calculates how far I am to the next rank (Column "To Next Tier"). Right now, I have it done manually for each achievement, looking at the rank it's on and subtracting. But doing this 15-18 times each time he plays the game is overwhelming. (Can't just drag down a formula because different things are at different ranks.)



Now to the Question:
I want to know if there's a way to use formulas conditionally? Could I tell Excel to see "if C3 is less than E3 subtract from E3", but "if it's greater than E3 to subtract from F3" and so on?

Can the if-then function be used with formulas, or only formatting/text? It's the only thing I've found that seems like it could work, but all articles/tutorials online only mention text or color. If it can only be done manually, that's tolerable, I just would like to know of a lazier option if it's available.