Electonics 101A teaches on to avoid connections when possible as each one is a potential problem.
As stated, ideally 0 V drop. In these cars with their multiple charging system connections and 60 years old technology I do not believe that is possible even with a brand new everything including harnesses especially at idle.
IMO a. 2 to.5 v drop is acceptable depending on the harness condition etc.

You also should be checking at 1500- 2000 RPM as most of the earlier and current (no pun intended) alternators do not reach full output under idle conditions. But that goes back to system load, pulley ratios and how fast the Alternator is turning.