The theory is simple.

As to mixture, idle is fairly rich compared to stoich.

It should have no real effect on afr if the idle fuel feed vs. vac is linear. All the fuel feed happens above the throttle plate, and the vac is mostly all downstream from the throttle plate.

The higher the idle speed, the greater the mani vac. Engine speed and thus vac is highest at the optimum idle mixture. You can use a lo range tach the same way.

In either case, you need a tach to set the idle speed. The process is repeated until you narrow down on both the correct idle speed and the correct idle mixture, since they are interdependent. You set the idle speed, adjust the idle mixture toward optimum which raises the idle speed, then reset the idle speed, then reoptimize the mixture. The amount you have to lower the idle gets less and less each iteration until they match (really, when the raised idle from the mixture adj fall in the factory spec idle speed range) and you have it dead nailed.