You can do it pretty easily by successive halving. 10%, just move the decimal over add 5% which is just half of 10% add 2.5%, which is half of 5% and add 1%, moving the decimal over twice

...of course you either naturally think that way or it seems like an unreasonable amount of work. YMMV.

Personally, I usually take 15% and add a bit depending on how good the service was.