I use 20% as a baseline,...then look for reasons to increase.
Very seldom do I decrease and even when I do...never below 15%.
I can afford it and gain no pleasure, as some people I've seen and known, in saving a couple of dollars...at the expense of those not as fortunate.
An EX friend made the mistake of sliding money back to me at the table and saying...
"you tipped too much." 
I also leave $3-$4 a night for hotel housekeeping.