What is your formula for tipping?

While out to lunch celebrating the mortality marker (bka birthday) of one of the Brethren, the subject got to tipping. Some people tip the server no matter what because they don’t earn that much. Some don’t want to tip at all because they feel that is what they get paid for. For others tipping is conditional. What formula do you use when tipping? If the food was lousy but the service was outstanding, will you tip? If the service was subpar but the food was off the hook, do you tip? Some other criteria you tip for or not, what is your formula?