Here is an example of where you want that gauge to read during a regular cruise..........
I do not use my gauge during accel only when at cruising speed. From there, I use the gauge to maintain 10-15 on the gauge. If I see the needle dropping below 10, then I am going up a hill or giving too much throttle. The readings from a manual vacuum gauge are instant and will help you control throttle output and save gas just like a Scangauge would.