[fortyonerag]

I think I see your point. If your 12V system drops to say 10V, the 6V meter will 'see' 4V.

Dropping to 8V, the 6V meter will 'see' 2V. Not exactly linear indication. More like a good/bad indication - idiot light.

Once the 12V system drops below say 6v, there will be nothing on the meter, but of course by this time the situation is code brown anyhow.