Look at the total
Move the decimal point one spot to the right
Half the number you see
Add the half back to the number (still with the decimal moved)
That’s your tip!
Dinner cost $43.15
You’ll notice that I round some at subsesquent steps. I like round numbers. This method is even easier if your service is crummy and you only want to tip 10% (do nothing to the number) or if your service is stellar and you want to tip 20% (double the number!).
This method makes it really easy to help split bills as well. You take the amounts that each person owes, and then add 20-25% to it for tip and tax depending on how your party is tipping. I err on the high side with large groups. One more example for 25%!
Dinner cost $15.99
Finally (2x the first plus the second) $4.00, so this person owes $20.
Now, you have the skills. You are ready. You can be the one that gets handed the bill after your whole group looks at it dumbfounded! Welcome to my world.