I always use the post-tax total.
My minimum basic, "no frills not great service" tip is 15%. Overtly rude service gets no tip.
I give 20% nearly all the time, and in cases of exceptional service (special orders handled correctly, service like new drinks without asking, good attitude/personality that adds to the environment, I have given up to 50%.
My two sisters are outstanding waitstaff, and I work in healthcare, which is (or should be) heavily customer service oriented. I know what good service is, and I think people should be lavishly rewarded for great service, if one can afford to do it. (some people really can't...)
That said...I am not surprised by liberals who appear on that site. To be the devils advocate, remember the name of the site...:)
If you can't afford to tip properly, then you can't afford to be eating out in the first place!!!!!!!!!!