So how are you seeing the pan vac?
external analog gauge or using a sensor for input to a box or..?

I did see a guy saying You need MSD part no 7762, some 1/4 pushlock tubing and you'll be fixed right up.
He says that he uses anytime he runs his car.
It records in psi, and you just need to convert that to inhg.
He just uses it to watch my vacuum trace for detonation.
Once it's powered up it starts watching the sensor, you don't need to do anything special
to make it work other than make sure the retard function is disabled, unless you want to use it, but it's referenced off of boost.

Do you have a way you do yours?
And where should the measurement be taken from