This is somewhat akin to using an "Entertainment Book" where one of the meals is free. The waitperson should not be held responsible for the acts of management (undercooked food, advertising promotions, etc.). I always calculate the tip on the gross amount*.

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* now having said that my wife and I have engaged in a years old debate of whether the tip should be based upon the pre-tax amount, or after tax. My premise is if I want to give a 15% tip for average service it should be on the pre-tax. The tax rate in my home county is 8.25%! Et ux argues otherwise.