I'd tried the "Merc Stick" first,  when I first got the bike.  They were unusable because the high performance cam on this bike made the mercury jump all over the place to the extent is was impossible to read.  Vacuum gauges of this type come with adjustable restrictor valves (circled below), and using them to dampen the movement was necessary.  It worked very well.

(Left) I performed the readings and adjustments at about 1100rpms which was about as low as I could get it to read with stability, and I closed the restrictor valves a good ways until the almost quit jumping (still just slightly vibrating).  You see, I was reasonably close, but the number one cylinder (looking low) was reading high and number four cylinder still jumping about 3/16s to 1/4 inch (after restriction-half inch before).

Update:  You may notice my #4 cylinder reads differently than the others and it jumps around twice as much as the others.  I finally and accidentally discovered the cause.  While cleaning the carbs, I was spraying them out with wd-40 just making sure the bleeder passages were clear, and noticed a tiny stream of spray coming out of the back side of the casing holding the main jet and emulsifier.  So its sucking gas straight into that area. 

The adjustment is made by undoing the lock-nut (green arrow) and rotating the slider adjuster (red arrow).  

Adjust three carbs to the forth, or lowest other.  That way you'll have plenty room for idle adjustment.  Only one thread should be showing above the adjuster.

The #4 is still jumping a bit, but now they're reading right, and I'm getting even better bottom end response and smoother idle.

(Tip) If you don't have a magnetic (couldn't find mine) screwdriver, place the port screws on the screwdriver as shown and move it (gently and slowly) into position.