Lordy, are you really trying to do this without ANY thinking or understanding of what is happening, and how you are effecting it?
Can you read the dial and see where it is labeled pressure and, on the opposite side, vacuum?
While connected to the intake, have you EVER seen the needle point to the pressure side?
Do you understand a 4 stroke cycle?
When the intake valve is open and the piston falls, do you understand that negative pressure is created, (with respect to outside atmospheric)?
The only thing in the path from air inlet to the piston that impedes pressure equalization is the slide.
Therefore, more open slide settings produce a lower vacuum reading in the intake runner than those of a closed slide.
The goal is to obtain the most closed slide setting, which should be the highest vacuum reading for the "master" carb, so as to indicate it is completely shut off for air entry. This would be the end travel limit you are seeking, that is never tested or obtained by using drill bits under the slides. Once that end point is found by inference of vacuum readings, the idle knob can be used dominate or set idle speed, and all other slides can be individually adjusted to match any vacuum level the master carb achieves on a running engine. When all slides equal the master, which has been proven to reach full bottom travel, then ALL the slide will have achieved the same capability. Then it won't matter how hot the engine gets, you can set idle speed with the idle knob, as it has full authority slide travel end point, all the way to off or engine stopped.
If this doesn't make sense to you or you can't understand what is happening using this procedure, perhaps you are better off yanking the carbs off the bike and doing a proper bench sync with full slide travel falling under your direct visual gaze.
Good luck!
