Question: how to calculate a tip percentage?
Answer: If you are tipping 15 percent, multiply the pre-tax amount by 10 percent and then add half of that amount. So, if you have a bill for $27.50, 10 percent is $2.75, and half of that amount is roughly $1.38. ... 
Another easy method is to look at your sales tax amount. ... 
A 20 percent tip is simple to calculate.