In article ,
Judah wrote:

I absolutely agree... I was just answering the question of why the math
"trick" works.

In flight I can't think about Math - I have to concentrate on things like
remembering whether I am coming from the East or the West!

Oh, that's easy. Fly at 4500 and call up ATC for flight following. If
he yells at you about your altitude, you know you're coming from the
West.