As far mixture settings we always adjusted to the highest RPM (assuming idle stop and timing are correct) then dialled it back in 1/4 - 1/2 turn.
As you run thru your adjustments you will find the point where it runs best and noticeably worse, the set point should be somewhere in between but a little closer to "lean" setting than at max revs setting.

Essentially it is a coordinated dance between timing, base idle and idle mixture.
If the hardened cap was still over the mixture screw then no one has messed with it.
If the cap was missing, then theres a REALLY good chance someone was in there.
Back in the day, it was pretty standard practice to pull the cap and richen the mixture a tad, they are set as lean as possible at the factory which works great when its all new, but after things run in a slightly richer mixture makes for a much better driving experience.