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.
I do that when flying GA type aircraft to size up distance on how far or how close; whether the aircraft is climbing my thumb or coming down - if climbing it’s coming towards me, conversely if its going down its going away.