James, I understand what you mean, but it really depends on the system.
On the Roush, see the three vacuum ports on the TB adapter? By design, they will NEVER see boost because they are on the INLET side of the blower. Yet they are a MANIFOLD vacuum source because they are BEHIND the TB. If you were to use one of them you would read normal idle and then the guage would stop at "0" under WOT. Huitt is right.
