Stephen, You're really asking two questions in one.
The most efficient AFR varies with load placed on the engine. The exact number will vary as others have pointed out. Generally as load increases, AFR should go leaner. Then somewhere around 80% load, it needs to get richer, much richer, so by max load its somewhere between 12:1 and 13:1.
Max load for a car will be 3rd or 4th gear, Wide Open Throttle.
On an engine with a hot cam, idle usually needs to be richer than a stock cammed engine.
13.2 will not be fouling the plugs (carbon covered), but less than 12 generally will.

The second part of your question has to deal with understanding the different circuits of the fuel curve. My advice is to get a copy of Urich & Fisher's Holley Carburators and Manifolds. No BS, no wondering if the advise is good or not. Urich was VP of Engineering at Holley, and the diagrams and explanations are top notch for a basic understanding of how any fixed venturi carb works.
You can find a g**gle book version, or go buy the real thing, used or new. Older editions just as good as newer.

Then you'll be able to use some of the tuning advice that others have posted in this thread.