I dont know how accurate this is or if this helps but I found this during some research, maybe you can find it useful
Idle 16" - 22" steady Healthy
Idle 14" - 20" steady Fair but worn
Snap throttle Jumps to 2" then on decel moves to 25" Healthy
Snap throttle Jumps to 1" then on decel moves to 22" Fair but worn
Idle 8" or less Vacuum leak, loose intake manifold
Idle Fluctuated between 14" - 19" Worn valve guides or head gasket blown between 2 cylinders
Idle Reading drops from normal Burnt valve, valve stuck open, misfiring spark plug
Idle 8" - 14" Valve timing incorrect or large camshaft overlap
Idle 14" - 16" Ignition timing wrong
Idle Moves between 12" - 16" Idle mixture incorrect
Slow engine speed rise Needle falls then rises suddenly Blocked exhaustWeak valve springs
3000 rpm Needle fluctuates and worsens with higher rpm Weak valve springs
Source
Using a vacuum gauge