Formulaic generosity

This wasn't my 2009 resolution, I started doing this in 2006, and I used this formula (I'm a bit of a geek, and I can't really do anything w/o hard figures and guidelines):

From net income I deduct 18% and save it, and take 2% and give it to charity (leaving me 80%). I do this in advance by picking two charities (last year I picked one and then let whichever charity 'charity mugged' me first have the rest), and setting them up for monthly payments - that way if my pay goes up or down I can change the payments accordingly.