Battery voltage drop

Grounded the 2nd field tab to the alternator case as suggested.

Starting with 12.15v on the battery with power off,

At fast idle, voltage read 12.95v.

Put my voltmeter to the green field tab at normal idle, i'm getting 10.25v, which seems high...

Since there's two field tabs maybe i have them reversed? How do I know which tab the green goes on and which tab should be grounded?

On the upside, when I put my idle back down to normal, when the motor was warmed up it was at least reading higher voltage than my starting, 12.55v... good sign?