We also have to think of the tax that will be applied. You got out to eat and order a $7.95 burger/fries and a $1.50 soda[1]... most people already know that it'll be more than $10. It is not so hard to do the math. Since tax around here is in the 8.5-9.5% range, most of the time it can be safe to just double the tax on the receipt to get roughly 17-19% tip... round up if you want to show extra thanks... down if not. Many places will also print on the receipt the various percentages of tip so you don't even really have to do much at all but some multi-digit addition. So unless one has a hard time with "carry the 1" the math isn't that bad. I tend to over tip so I can get my grand total to a nice round number. And it is not uncommon for me to add an extra buck or two if my kids have made a huge mess of the place.

[1]But don't get me started on the extortion of $1.50 for a fountain drink.