I prefer taking 10% and doubling it for a 20% tip.
Take the total, moved the decimal one digit to the left, double it and round to the nearest dollar. I round up at the beginning and end as well which is either close enough or a dollar high if the change is small enough.
$138.72 > 139 > 13.9 > 27.8 > $28
Close enough!
Neverclear@lemmy.dbzer0.com 4 hours ago
Or just take 10% of 138.72 and double the result… 13.87 x 2 = 27.74