Restaurant servers generally seem to be an underpaid and under-appreciated bunch. So I try to be a good tipper unless something horrible happens. By doing so, I'm helping out someone who has a tough job and, selfishly, I'm making my life a little easier by not having to do so much math. When in (mathematical) doubt, I always round up.
So the first step when the bill comes is to figure out how much to tip. Let's say the bill is $38.69. Don't even ask me to try to calculate what 15% or 20% of $38.69 is. Instead, what I do is look at the amount of sales tax on the bill. In this case, it would be $3.19 because the sales tax rate where I live is 8.25%.
So if I double the amount of sales tax on the bill, that will give me a 16.50% tip which is within the acceptable range. Doubling $3.19 in my head is easy enough, even for me. Three times two is six, and I would round the 19-cents up to 20 since doubling 20 is easier than doubling 19. Are you still following me?
Therefore, my first inclination would be to add a tip of $6.40 to the credit card receipt. But I can't stop there because, remember, the total bill amount is $38.69, so I wouldn't want to have to to add 40-cents plus 69-cents and "carry the one" in my head (or on paper). So I'll save myself the trouble and just knock the tip amount up to $7 and not have to mess with the cents column at all.
As for adding $38 plus $7, I can do that easily on my fingers if needed (and with my hand in my lap or under the table so nobody notices). Voila! The server would get a $7 tip, and my total credit card bill would be $45.69.
It's a convenient, albeit sad and pathetic, way for a math hater to calculate a tip. Any questions?