We now have TiW so this is no longer an issue for us, they automatically add 18% tip. This is how we used to determine. It depended on the service and if it is sit down or buffet.
15% for for Buffet.
15% for poor service
18% for good service
20% for excellent service.
I have also heard that you should just double the amount of tax to get how much to tip.
Where I am (hour north of NYC) it's about 7.375% tax or slightly more depending on if you're in a city. NYC is 8.375%. So doubling it would bring it to close to 15 - 17%. Of course in other places that probably won't work.
I guess for mental math... The easiest way is just move the decimal and double. Then you have 20%.
75.46 = 7.55 * 2 = 15.10
15% then would be 7.55 / 2 = 3.28 + 7.55 = 10.83
For 18% it's a little tougher I guess, unless someone knows a trick. Closest I can think of is taking 20%, taking 15%, getting an average.
Otherwise possibly take 1%, .75 in this case, multiply by 9 then times 2 or if you're really good then by 18. Of course this takes a little more work and cuts off some change. Of course you could always just round up to the next dollar.
0.75 * 9 = 6.75 * 2 = 13.50 or 0.76 * 9 = 6.84 * 2 = 13.68
0.63 * 9 = 5.67 * 2 = 11.34
etc.