The term "dwell" is an expression of the point gap in terms of the number of degrees of distributor rotation that the points are closed, or "dwell together". A dwell meter is actually a voltmeter- - - -the scale is just different. Anytime the points are closed, the voltage at the downstream side of the coil is effectively zero, because one side of the points is grounded. When they're open, the voltage present at the "dist" terminal of the coil is charging system voltage, basically an open circuit from the battery, through the ignition switch and the coil primary winding, with no current flowing. The dwell meter is reading an average of the "points open" and "points closed" voltage at the distributor side of the coil.
Normal dwell is around 66% of the available distributor cam rotation, which is 60 degrees for a 6-cylinder, or 45 degrees on a V-8. A V-8 running at 30 degrees of dwell is using up 2/3 of the available rotation of 45 degrees with the points closed- - - -exactly a 66% "points closed" reading. 2/3 of 60 degrees on a 6-cylinder would be 40 degrees. Just for grins, try putting a voltmeter across the circuit from the distributor side of the coil to ground with the engine running, and compare the voltage you get to the above little math exercise. Then do the same check with a dwell meter. It will be pretty close!
Jerry
