Pump it while you have coffee
Brother of the Benz
You are really taking the pressure you can't read to an extreme.
And yes the minus pressure scale on my manifold set is of no real value while evacuating, as time is the controlling element.
With time at 20-29"hg, water will boil and this vapor will be removed by the pump.
What is a realistic time, ?
I pump a unit down that has been opened to the atmosphere at least 30 minutes.
If the system has lost it's gas, just long enough to introduce enough gas for leak testing.
The prime use of the vac side of the gauge is for rate of rise. How long will the system maintain this inacurate pressure and will the needle creep to atmospheric.
So you see that portion of the gauge really has 2 functions.
Try this thinking and I believe you will be more at ease.
Happy Trails Beep Beep from The Spiderman in Houston!!!