One of the quick ways of calculating a tip was to take the tax (8.25% around here) and double it. Or take the whole bill and move the decimal back one and double that. Which would be 20 percent. For example, $80 means a tip of $16.
I always like the staff at Bill Smith Cafe, for example and just rounded up to the next denomination of currency. 16 becomes 20. Go back and they remember you.