Meme-Free Zone

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...

According to Private Eye it now costs £1600 in performance rights fees to provide piped music to a 10,000 foot area, e.g. a department store. Marks and Sparks just dropped muzak, presumably for this reason.

It occurs to me that there's probably a lucrative market for anyone who is prepared to write a computer program that will generate random music that isn't subject to performance rights payments for this purpose and for e.g. call waiting. All evidence suggests that quality is VERY low on the agenda of the people who use this stuff, so it doesn't need to be GOOD music.

Is this an unfilled market niche, or are there possible snags - such as Performancee Rights Payments still being charged and going to the programmer?