Did you get the conventional 123 or did you get the 123 'tune' version? The 123 tune version allows you to program any advance curve that you want. Starting with pure speed based advance from something like 10 deg - 1000 RPM rising to 30 - 32 degrees around 3000 RPM with no vacuum component is a good starting place. You can add vacuum advance if you are inclined.

I have a B20E running on MSExtra. My engine cranks with an ignition advance of 10 deg for fast start; but, idle advance (900 RPM) is at 15 deg. That seems to give the most stable idle and lowest idle MAP. I started out with straight engine speed based advance peaking out at 32 deg around 3000 RPM. I have been adding in some MAP based advance to see if that improves fuel economy (its going to be a real long term test period). If you are having trouble with the settings, finding a curve that gives you a pure speed based advance should give you an engine that operates well. My recollection is that some (all??) of the B18 engines had a pure speed based advance curve.

I would stay away from the E (and F curves) if you are not using the B20E / F manifold. The B20E and F distributors had a vacuum retard function which was further complicated by the fact that the E and F did not use a straight manifold pressure reference. The distributor pressure tap came off around the throttle plate (rather than manifold proper) and I think it may have been right on or upstream of the throttle plate so the distributor was getting a manifold pressure reference that was affected by the position of the throttle plate.