In a reciprocating engine the fuel mixture is naturally aspirated through the carburator then distributed to the combustion chambers via your intake manifold. As the engine speed increases with the throttle being opened more, the engine vacuum will decrease due to the stabilization of the engine's negative pressure with atmospheric pressure.

As a general rule of thumb, manifold pressure is an indication of how much power your engine is developing.

So take it from a Frenchman Junkyard Frog, what you've got to do is hook up a manifold pressure indicator that indicates in increments of inches of mercury in relation to a standard day at sea level (29.93" Hg.) and you're all set.

Big Lifter

Tuna is the chicken of the sea
Iguana is the chicken of the tree
Chicken is the chicken of the three