I don't trust the Kerry source
1. It's a site meant for weird facts, not a news organization
2. It was submitted by "Mike", who is "Mike"?
3. They don't even know how to quote correctly. Look closely, there are several instances of missing opening or closing quotation marks.
I really am interested though, so if you find a different source let me know.
Edit: Doing the math, the Bushes pay about 23%, which is still less than the average worker.