One of those mysteries of civilized life that nobody completely masters is the art of leaving a tip. For those unsure of how much to press into a porter's palm or who go into mathematics anxiety trying to compute 15 percent of a restaurant bill, there's a Pittsburgh-area firm ready to come to the rescue. Their answer is a plastic, credit-card-sized guide to the ins and outs of tipping, covering various services on one side and percentages on the other. Three of them can be had for $2 from ABELexpress, 1000 Washington Ave., Carnegie, Pa., 15106. Their recommendation in case of bad service? Leave a quarter. It shows that you're displeased, not forgetful.