Same as everyone else... 1$ per beer. If the bartender under-pours, I will usually tell him/her "go easy on the head". If he/she keeps doing it... The tip may disappear. And likewise, if I get a beer or two on the house, his/her tip goes up.

I've had a bad run of under-pours in the outskirts of Chicago. Some brewpubs even charge for a taste of beer (free tastes are common in brewpubs). That's really bad form. I've also found quite a few places that sell 14 oz "pints", and have the b*lls of putting 1" of head on top of the beer.

Makes me wish the US had the same rules as in the UK for measured pours!

JP