You take 10% of the bill amount, because that’s easy, and you double it. Problem solved.

Back when it was 15% is normal, 10% of the bill, half the 10%, then add those numbers together.

You’re welcome, CNN.

Not to say you can’t or shouldn’t tip more than 20%. Just saying the standard customary tip is like 20%, that’s the bare minimum to not be an asshole.
@[email protected] - Goes both ways. I have left a 100% tip on a dinner check for outstanding service - even where there was an 18% customary tip already included. I also write on the check - "Outstanding service provided!" But I have also actually gotten up and left, with no payment if the server was notedly rude or just completely inattentive. (I do pay the food bill for any food eaten.) But I won't go back to that location again either (not even for takeout), and if needed, will leave a bad rating at review websites to warn others - and for the managers to follow up on, if they read it.