You'll get a better signal from port vacuum. Some people use the manifold vacuum port, to try and run a distrbutor vacuum advance. I don't know why. The port vacuum, is designed for the advance mechanism. IMO
Ported vaccuum signal wil rise from "0" to the same as manifold vaccuum at about 15 degrees of throttle opening. It will mimick manifold vaccuum from there on up.
VENTURI vaccuum will rise as air speed becomes greater reguardless of manifold vaccuum.