How do we calculate top of descent (TOD) in flight? What is the in-flight trick?

I know how to calculate it in theory, but how can I calculate it fast? For example, I know I can use this formula for calculating the rate of descent:

[ground speed + add 0]\2

I'm trying to find a formula like that for TOD.
Any approximate method is going to vary for different types of plane, depending on their speed, altitude and descent rate, but here's how I figure it for my Cessna 172.

I know I'm traveling around 2NM a minute (actually a little slower), and I like to descend around 500 fpm.   

So if I have to drop 5,000 feet to pattern altitude, the TOD is 10 minutes out (5,000 feet / 500 fpm).   At 2NM a minute, that is 20 miles out.

So if I have my GPS / DME set to my destination airport, I am looking for both ETE: 10 min, and also Dist: 20 miles, then I start my descent.

Its approximate, but its pretty close for planning purposes.
Its always just:

[Altitude Drop] / 500 = Minutes
Minutes * 2 = Miles