The main reasons for doing a bench sync are twofold. First to get all the slides approximately the same height but more important, to ensure that you can move the slide UP or DOWN; that is increase or decrease idle speed with the idle knob so you can adjust your idle up or down. I personally think the drill bit trick is pretty much a bad idea because if you use a larger diameter drill bit, while the slides will all be the same height, you will end up backing the idle knob all the way and won't be able to get the idle speed down to where it should be.
Try reading your factory service manual for the correct procedure. Essentially, you turn the knob in so that the flat spot on the bellcrank and the stop on cable holder bracket have about 1/8" between them. The purpose of doing this is to get the idle knob screwed in just a bit so that once you have adjusted the slides, you will be able to then move it up or down within the range where you would want to adjust idle speed. The next step is then to adjust the slides to that all of them are opened the same amount. Just eyeball it where the slides are about 1/16" open. Try closing the slides with the idle knob and opening... The correct idle speed will be acheived with the slides open about 3/16" or a shade less so you should be able to move the slides up or down with the idle knob when the slides are at about 3/16" open.
If your eyes are good, you can typically bench sync so that when you put a manometer on the bike and sync with this guage, the guages are no more than 5 PSI vacuum different... This makes it easy to tweek the adjusters and get them all the same; usually between 20 and 25 on your gauges.
