Vacuum readings vary from engine to engine. Normal would be 17 - 22 in. at idle. It will go up slightly as RPM's do and spike high when the throttle closes. Vacuum should be very steady at idle. If cam timing or overlap is wrong, vacuum will be lower but steady (10 - 15 in.). Late ignition timing will also cause vacuum to be lower. In our cars ignition timing is not adjustable but it is tied to cam timing via the cam position sensor. Wandering vacuum readings usually are caused by vacuum leaks. Bouncing readings are caused by mechanical problems. Plugged cats cause vacuum to fall as RPM's rise.
Read several vehicles to familiarize yourself, including well running ones and other problem ones if you can.
