When "friends" split a check, advice needed on getting them to fair share

[rjmacs]

I generally put in what I owe plus tax plus tip plus a couple extra bucks, and if/when the check comes up short, ask that we review it as a group. When I reveal my contribution (I neither drink alcohol nor eat meat), I just look at the group and say, "I had the grilled cheese and diet coke, so I put in $15, which covers my food, tax, and tip plus a couple extra bucks. I can add more, though, if we're still short."

I haven't ever had anyone look me square in the face and say, "Yeah, rj - you should pay more."

I'm all for being overgenerous when it comes to sharing costs, but that doesn't mean being (intentionally or accidentally) abused when the bill comes.