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I think the readings seem slightly low.
My Echo reads about 21 at idle and it reads slightly higher under similar circumstances.
It's a manual and has a very low idle speed, it will be higher for an auto in gear, or any car with a higher idle speed.
The fluctuating gauge needle is an indicator of one of two possible problems.
It could be an engine problem, but I would not assume that to be correct yet.
Its more likely that you have tapped the vacuum near one of the individual runners in the intake manifold. I can't really tell from the photos.
You need to tap the vacuum from the intake plenum, where all the manifold runners meet.
My preference is the hose to the brake booster, which should connect to a steady vacuum source. If you try that and it still fluctuates it might indicate there is a problem with the engine, induction, or ignition systems.
Idle vacuum is also a function of idle speed, and engine compression. My Echo is 10.5 to one so it reads fairly high idle vacuum.
Also understand that your altitude makes an difference as well as atmospheric pressure.
Higher altitudes and lower atmospheric pressures will reduce idle vacuum readings.
Otherwise your ranges seem to closely match mine, just slightly lower.
My 76 Z car would do about 18 at idle, but it's compression ratio was 8.5 to 1.
regards
Mech
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