Obviously John Gruber doesn't know enough math and MAY need a "tip calulator" as Ryan Block suggests:
From Daring Fireball:
Helpful ‘Tip’ for Ryan Block ★
From page 3 of Engadget’s massive iPhone review, regarding the Calculator:
Missing from the iPhone, though, are dedicated tip and scientific / graphing calculators, or, perhaps more usefully, a tip calculator.
If you know enough math to even know what a scientific calculator is, why would you need a “dedicated tip calculator” mode? Take the total and multiply by 1.2 for a 20 percent tip.
I sent him a correction by email almost 14 hours ago, but the post remains unchanged.
If I take a sample bill from a Mexcican restaurant two days ago and multiply by 1.2 ... here's what I get:
(I did that math on an iPhone)
But that's not the only thing wrong with his equation. Here's the email I sent:
Just a note ...
Figuring the tip IS NOT taking the TOTAL bill and multiplying by 1.2 ... it's taking the totalBEFORE TAX and multiplying by 1.2 for a 20% tip with total included.
If you do it by the taxed total you are actually giving the waitress 2%-3% more.
Waitresses are given whatever amount they receive above the pretax total minus the tax.
Meaning; a server pays the tax total BACK OUT each night from her pocket. A lot of restaurants do this for the server, but it's a common mistake and a thing most people do not know that they are overpaying. Particulary when you mean to give a 15% tip if the service was bad or mediocre. Most times you are still leaving 17%+ if you leave a tip on the taxed total.