probably measure the vacuum after the throttle body. When the throttle flap not fully open, it will create a vacuum behind the throttle body. At wide open throttle, the air is free to flow and should have pressure approximately around 0 (atmosphere pressure).
It probably helps to tell how effective your intake system is. If the intake system is tuned, you could achieve pressure a bit higher than atmosphere....
Don't think it help for dialy driving, except when there is a leak in the system....
