[SkyDancer]

Rule of thumb: In English, rule of thumb refers to an approximate method for doing something, based on practical experience rather than theory. Such a rule is usually a principle with broad application that is not intended to be strictly accurate or reliable for every situation.

This usage of the phrase can be traced back to the seventeenth century and has been associated with various trades where quantities were measured by comparison to the width or length of a thumb.

Typical ignorant liberal colleges.

[HartleyMBaldwin]

You beat me to it, because I took the time to haul out the OED. Nothing at all about wife-beating in any of the definitions or citations of the phrase “rule of thumb”.

[DUMBGRUNT]

Rule of thumb!

Don’t waste any time on this nonsense.

Here’s the real deal, you might need it if in a very bad spot:

You are being tracked and have limited concealment a light aircraft is circling overhead.

You have a rifle but only 100 rounds that may be needed later.
You do not want to waste ammunition but can only guess the distance and it can be deceiving.

HERE IT IS:

Hold your thumb out and up at arm’s length, like an artist, but viewing the aircraft.

If your thumb covers the aircraft, SAVE YOUR AMMO.

If your thumb does not cover the aircraft. GO FOR IT!

Just a bit of simple math and I hope you never need to use it.