Get there earlier and then you can pay

I waved him off as he was reaching for his money.

"Why do you always so that?" I always do that because the beers are $1.50.

"Anything you throw in is for tip" I said as I showed him the bill and the cash that was already collected.

"Is that right?"

"You know how I do math," I said mostly as a joke but also to let him know it was a large tip and on purpose.

"That's like a thirty dollar tip."

I just shrugged my shoulders. We have a system when we have commitments later in the evening: ten bucks per person if we're just casually drinking, twenty if we're eating and drinking which usually leaves a tip over 50%.