My hubby doubles the tax and then rounds up to the next whole dollar. So say the bill is $20 pre tax then the is the tax is $1.75. He doubles it to $3.5 then goes up to $4.00.
I figure 15% of $21.75 which is $3.26 then I go up to the next dollar $4.00. I never leave change in a tip. As a hair stylist I always hated getting change to easy to lose.
My hubby feels his method is easier. I like to do the math in my head it keeps my brain working right.