I try and tip 20%+ but sometimes the service does not warrant that so I do adjust.
Now throw this into the works: You go into a resturant (using a coupon (hey if they didn't want you to use them they would not print them)), don't forget to tip on the original amount not the adjusted amount.
Had a funny thing happen to me last Friday, Took the grandkids to Ruby Tuesdays (they love the place) and the waitress brings me a glass of coke to refill their's (we get them the "to go cups with lids and straws"), I set the glass down, decide they do need a refill and pick up the glass. The dang bottom FELL off of the glass,coke everywhere, none in the food but I took a coke bath and so did one of the girls. She came over, helped us get it cleaned up, got us a new glass and then went a raised cain with the manager (I know this because he came running over to make sure everything was ok). Now after everything was said and done they knocked of a 10 spot off my bill, and after taking my coupon off my bill was only 31 and change for 4 people. I left her the normal tip for the simple fact that the glass breaking was not her fault, now if she had poured a glass on me that would be different, she was very prompt on getting everything cleaned up and even offered to move us to a different table.