Low charge to no charge issue pulling hair out

Dana67Dart said
Just went out and tested my 67 dart.

Last driven yesterday or Tuesday for 30+ minutes.

Key off, doors shut, no accessories on.

Battery plus to minus post 12.66v

Alternator output post to Battery minus post 12.66v

Key off, doors shut, headlights on.

Battery plus to minus post 11.96v

Alternator output post to Battery minus post 11.53v

0.43 voltage drop

Im thinking my ammeter is the culprit evwrything is good up until i get to the post, when i get to the red post on back of ammeter thats where i get the big drop from 11.8 to 10.30 giving me a 1.5v drop