Well I am sorry to say that whatever method you use to set the idle timing which involves manifold vacuum, it will result in timing more advanced than factory figures. Manifold vacuum is an indicator of volumetric efficiency and will generally go up with timing and timing induced RPM increase.
As I said before (like a broken record), the idle timing requirements and the load timing requirements are rather different, but when you offset one, the other gets the same offset whether it needs it or not.
The fuels that were available back in the 60s were more volatile and have a higher octane rating than todays pump gas.
Use whatever method you think will work for you (its your car), but nearly every rootes engine I've torn into has broken rings and pock marked pistons from long term over advanced ignition timing.
All my alpine engines have been rather modified, to the point where the factory distributor curves are not even close. As a result I always recurve the distributor and thus have control over the idle and timing under load.
The method I used to set the curve is either an accelerometer based setup, or an RPM logging setup, both of which show the equivalent of the torque curve of the engine and allows me to decide if my change was helpful or not.
A simplified though single dimensional equivalent involves a stopwatch and two RPM or speed points with a wide open throttle in a gear high enough to provide accurate timings.
Even this simplified method is superior since its the timing under load that kills an engine with detonation (ever heard pinging at idle?), and wide open throttle timing is the most important attribute to both power as well and determining if the engine is detonating.
As you add timing (at wide open throttle), your acceleration will get better with each degree of timing until it hits a plateau. The plateau usually is across two or 3 degrees where more timing doesnt help acceleration nor does it cause knock. Once you find the plateau, you set the max RPM timing to the onset of the plateau and you are done.
You record the idle and max timing figures as indicated by a timing light, then if you feel the idle and light load timing is not optimal, you can tweak the timing to "offset" from best power timing and determine if the advance rate needs tweaked. Again measuring the "offset" in timing on the timing light.
Then with both sets of timing figures you can determine what the curve should look like and modify the distributor for that curve.
After which you would set the max RPM back to the same point where the plateau just began.
Yes its complicated.
