I thought most people tipped on the pre-tax, not the post-tax amount?

I don't mind when they precalculate 15-18% on the post tax as long as the service is good, though, as I usually tip 20% for good service and while the tip amount goes up somewhat it is still less than 20% I'd tip otherwise.

What I WON'T do is tip up to 20% total if they precompute it.

If "just OK" service that I'd usually tip 15% on the pretax, it's a percent or two, and I'm not going to make a big deal about it except on a very large bill.

If it's worse than just OK, I might well talk to the manager.