Did Your Waitress Screw You Over?

You know how, at the end of a meal out, you spend a few painful seconds deciding on the appropriate tip? it might not matter.
This happend to me in a restaurant somewhere in Texas. After disappointing service, I left a 10% tip, about $2.50. The waitress, it seemed, thoguht a $4.50 tip would make her happier. And you did notice they swipe the card *before* you write the tip? which means, of course, that they -manually type in- the new total amount. Guess which number she typed in?
A month or so later, reviewing my credit card bill, I found the petty theft. It was obnoxious. Of course I got my $2 back, but hey, this would probably go undertected with most people. Probably does.

Why do I mention this -now-? recently I found a very simple way to keep your mind working while ensuring this is not likely to happen.
What I do with restaurants now is use the cents digit as a 'checksum' digit.
To do that, I sum up the digits (not including cents), and take the singles digit as my checksum. An example would explain this:
Original amout including tip: 24.32
Checksum: 2 + 4 = 6
Amount I write down: 24.326

Original amout including tip: 29.18
Checksum: 2 + 9 = 11 (use '1')
Amount I write down: 29.186

As you can see, if a waitress/waiter/whomever writes in a whatever number, I have 9/10 chance of spotting this. And if they simply (as most people would do) add a dollar or two, the checksum would -always- not match, alterting me to this.

Of course, if they changed the checksum digit, they'd be able to fool me. But people with the brains to do that (and the knowledge of my system - whcih is now public, but still) have the brains not to risk jail time over a theft of a few dollars.

if you have javascript enabled (most people do), I created this calculator - enjoy