Each notch is usually 2 degrees but don't go by those.
Set the initial timing advance where the engine starts and run best. That is usually about 16 to 20 degrees BTDC on a 1971 and later low compression engine with dished pistons. The 1970 and earlier high compression engines with flat top pistons can only tolerate about 12 degrees BTDC initial timing advance before the engine tries to compress the ignited mixture.
As was mentioned above, the initial timing should be set with the vacuum advance disconnected and the vacuum source plugged. I will add that the idle speed should be turned down as low as possible (600 RPM) while the intitial timing advance is set in order to prevent the centrifugal (mechanical) advance from working . All you are setting at the harmonic damper is the timing at the crank. After the timing at the harmonic damper is set, re-connect the vacuum hose to the distributor vacuum canister and turn up the idle to specification.
It is difficult to set the timing in this manner if the engine is equipped with a radical camshaft that causes the engine to have low vacuum. In that case, the engine will not idle low enough to prevent the mechanical advance from working and a different method must be used.