[mark houghton]

Adjusting to 3% is all fine and dandy, but if you're trying to pass an emissions test then that will be way too rich. It's not a "magic number", but rather dependent upon your car's build and general state of tune as to how well it will idle at 3%. Lacking any hard data (such as an emissions exhaust sniffer or an AFR gauge) then let your ear tell you how she's idling as you adjust the mixture screw.
I idle at about 13.8 to 14.0 which my car seems to like. Been a long time and many adjustments since I "sniffed" with my old Gunson Gas Tester, but I seem to remember something in the neighborhood of 2.0 to 2.5% CO works well for my car.