Oh, with drinks it's even harder to calculate in the US. The 'typical' tip, regardless of (a reasonable) price is $1 flat. For a $3 domestic, that's a ~33% tip. For a $7 microbrew that's ~14% tip. Even with $10 martinis, $1 is 'typical'. Trying to factor that all in to the tip on the bill makes for a heck of a problem, especially after a few drinks!