derek uk covered the procedure for carb balance well. A piece of vacuum hose works for me. The motor should be warmed up thoroughly and the "choke" off completely.
I do take issue with your procedure for the mixture adjustment. After getting the balance and idle speed done and the linkage locked you can use a tachometer to achieve the highest idle rpm by adjusting each carb mixture control-- but leave the mixture adjuster alone at that point. When you lift the testing pin on each carb in turn you should have a momentary rise in rpm (one or two seconds) and then a drop off (the 200 rpm you site is fine). The motor should be able to continue running at that lowered rpm as long as the pin is raised. Check each carb in turn. If the rpm should drop to near stalling -- set the other carb a flat or two richer. If, instead, the rpm should rise -- set the other carb a flat or two leaner. Then go to the second carb and do the pin lift as you did with the first carb--always making the adjustment to the "other" carb. When you get a 200 rpm (or so) steady drop at idle by testing each pin you should have a proper mixture. Be sure the dashpots have oil and then road test. -- Dave