on my 350/auto/2 barrel, I have always set the dwell using the non-meter method "turn until it mis-fires and then back a half turn" as described in the pontiac shop manual.

In other words I have never known the actual degrees of dwell angle

Last night I bought a Actron meter from Autozone.
I hooked it up and I had 20 degrees of dwell.
So I adjusted it to 30 degrees as per the pontiac shop manual( all V8's) , and it idled like crap, stalled and would not restart.
So I turned the adjustment back and the car started and seemed to run best at about 21 degrees.

Is the meter wrong or is some other thing (plug gap, timing chain slack, valve lash) out of adjustment