Chris-just so you know I am NOT a tightwad
My last dining check came to $63.I tipped the waiter $20 because he did an OUTSTANDING job under difficult circumstances-so that is what?35% of the pre tax(the amount you properly should tip on)bill.

Now-as to the auto 18% I think management does this to assure servers get a better than average(15%)tip because tables of six or more are more work and when a party split a check the server often gets shafted by one of the six=solved by the auto tip.

Of course you can tip more-you can always do that to reward excellent service-but you should call any shortcomings in the service to the managers attention and ask that the tip be adjusted.