Esimate distances with your arm and this rule of thumb

Nice!

And here's a little story for you. After reading this, I thought I'd try it out. How far am I from my screen? Hold out thumb, switch eyes. It moved about an inch. So I'm 10 inches from my screen? I'm holding my arm straight out, and it doesn't touch the screen. My arm is certainly longer than 10 inches. What's wrong? Ah, this rule gives the distance of the object from my thumb, not from my eye. Sometimes the two are significantly different.

In order for this to work, you have to accurately estimate the width of your target.

In many situations you'll be able to find something close by whose width can be estimated -- perhaps a door, a car, a tree...

Doh, another one of these binocular-visual-only tricks. Racism!