Plymouthy,
The distributor turns at 1/2 of crankshaft rotation.
One complete cycle for all 6 cylinders is,in fact, 720 degrees.
However, in dealing with dwell and distributor rotation, we are dealing with one complete revolution.
There are 360 degrees in a circle.
There are 6 cylinders in our old flathead motors.
The distributor make one complete rotation to fire all 6 cylinders.
Dwell (or, more accurately, dwell angle) is the number of degrees during which the points are closed.
The correct dwell for a 6 cylinder engine is 40 degrees.
6 times 40 degrees equals 240 degrees.
360 degrees minus 240 degrees equals 120 degrees.
120 degrees equals the length of time, expressed in the degrees of a circle, that the points are open.
120 divided by 6 equals 20, which is the correct gap for the points in thousandths.
Corrected thanks to Plymouthy.