Surely the calibration technique depends on what or how you are trying to measure?
If you are measuring actual values of vacuum, then it's important to get them zeroed properly.
If however you're just trying to balance them, it doesn't matter whether the needle starts at 0 or 100, what's important is that all the guages read the same value for the same level of vacuum.
Therefore, in this argument, I side with Daviee.