Ammeter works when key is off?

Oh. I see.
My bad for not reading the article.
You might want to put in a volt meter just to better keep an eye on things since that would "bypass" the AMP meter for charging current, I guess.
Maybe hang a small one under the dash in front of your left knee.
You know, a some people have run a wire through the bulkhead connector and eliminated the Packard type blade connectors for that load you are talking about.
But that probably is more wiring than you want to do.

That meter melt down is a point of big debate.
I think as long as your connections are good and tight on the back, the AMP meter is pretty reliable.
That's my experience, anyway.
Yes planning on little digital volt meter also