I would not pay much attention to the marks. Just set using a vacuum gage and road test. As mentioned above, just shy of pinging and hard starting is probably the best.
Also, a common problem with older carbs is the power enrichment circuit. If the spring is not the optimum tension it will richen the mixture under lighter loads than needed just killing mileage.
From your description of the plugs being rich I would bet on this or a float level problem.
