You can use a vacuum gauge if you like, I am not arguing that point. But what makes good vacuum? The faster the engine runs, with the butterflies closed as much as they can be.
What I am saying is you can "hear" what the vacuum guage would tell you. You adjust the the mixture screws till it runs the smoothest/fastest, and then adjust your idle speed if you need to. Then you adjust the mixture screws again, and tweak the idle speed if needed. When the engine runs the smoothest and highest rpm, and is at a reasonable idle speed, that should be the highest vacuum reading. Tuning by "ear" like this gets you very close.
1989 F250 XLT Diesel