ffutures | Setting up a spreadsheet

This is probably simple but I'm drawing a blank on how to do it.

When I get the revised web site up I want to pass on some of the takings from the tip jar (if any) to Cancer Research UK. My current plan is 10% until I've covered the annual cost of the site, then 25%. To avoid micropayments the way this will work is I'll wait until £50-100 accumulates, then pay it to the charity, plus interest, as I did when it was shareware.

What I want to do is create an Excel spreadsheet where I enter the takings and it puts 10% into the charity column until I pass the annual site costs - currently about £85 including domain registration fees - then puts in 25% of each subsequent transaction. At the end of a year I want to reset the counter for costs and pass on 10% of subsequent transactions until my takings are over the site costs again. And so forth.

Is this possible, or would it be easier to change the ratio manually? This will of course be academic if I make less than the site costs, but I might as well think positively...