You set your 'advance' with a timing light..not by idle speed

My old man taught me to set the A12 idle by turning the distributor each way, one way slows (Retards), the other speeds up (Advance). Put it in the middle. This recommendation is 'backyard boys' at it's best.

Get a timing light and set it to the factory setting.

97RON should be fine with a regular head shave.

Set the idle timing first, set the idle to 800-900rpm.

If there are still problems, address them. Saying that the 'engine is old' and setting idle higher is a way of avoiding essential repairs, or inappropriate modifications to the fuel/ignition systems.

Chris