Steven P. McNicoll wrote:

Let's take a different tack, maybe this will help.

Yes, great idea.

Take an altimeter and set it on a table. Don't move it!

Turn the knob that adjusts the pressure scale to one extreme. Record
the indication on the face of the altimeter. Now turn the knob to the
other extreme of travel. Record the indication on the face of the
instrument. Now the big question: which value recorded is the altitude
at the level of the instrument?

Matt