An interesting 'rule of thumb' that depends on human anatomy that works for astronomy and other 'rough guide' measuring.

Hold out your arm with with thumb up. Close one eye. The angle subtended by your thumb is now 2 degrees. There are other fixed numbers for the palm out with fingers closed and palm out with fingers outstretched, but I can't recall them at the moment.

It is now believed by some archaeo-astronomers that is how neolithic astronomers measured the sky. Funnily enough that seems to be why there are 360 degrees in a circle...