Cylinders #3 and #6 have a single throttle plate mounted to the throttle shaft while cylinders 1 & 2 and 4 & 5 have two throttle plates per shaft. The long shaft and the short shaft are connected for rotation using a shaft coupling which is either a cylindrical one or for the earliest Webers a "folded spring". If the shaft coupling is broken or loose or has been "adjusted" by a previous owner then there is the real possibility that your #6 throttle plate is mechanically too far open to be adjusted with air correction screws.
Typically when "Lean Best" idle mixtures have been achieved you will see a STE reading of 4.5 to 5.0. Larger air flow readings indicate a rich mixture that requires more air flow to allow combustion.
Your air flow reading on #6 is large (8 per your posting) and your other cylinders are at 4.5 (6 @ #5 isn't that far off) so I'd check for a throttle plate that isn't closing at #6. Closing the idle air correction screw decreases the air flow at idle speeds.
Check out this site for another version of basic tuning procedure:
Weber Carbs Technical