what do you consider to be "net income" ??


If you look at your 2009 Form 1040,  line 37 - Adjusted Gross Income - should be comparable to take-home pay for the "city folk".
This number takes into account health insurance, 401K, etc, etc, and should give the best comparison between those who have a weekly/bi-weekly paycheck, and those who have income primarily from a business.  I would start with this number and then decide what % you can afford to give from there.