Paul - The dwell / tach only wants a pulse signal from the ground side (-) of an inductive ignition coil to operate - probably might ought to avoid any capacitive discharge ignition units (MSD etc.) - but the HEI and Duraspark II types of "electronic ignition" are just substituting a grounding transistor for the points.
If you switch the unit to tach, it counts the pulses and indictates one revolution for every four times the coil "fires" (for a V8). Most of the units have a switch where you can select the number of cylinders - 8/6/4 so the gadget knows what to use for the divisor.
The dwell is "calculated" from the portion of time the switch is closed between each time the system cycles. On the early electronic ignitions the dwell was set to a fixed amount (by a calculator timing chip) but later versions (GM HEI) used a more powerful "brain" to alter dwell as the rev's increased (and thus give the iginition a little extra zip). Because our V8's fire every 45°, the ratio of time "on" to "off" compared to the time "on" to "on" can be displayed as degrees. For instance if the grounding transistor (which replaced the points) is closed and operating 50% of the time between "on to on" cycles - the dwell would be shown as 22.5° (half of the total angle).
This number is a little too low for a traditional "hotrod" inductive ignition - it is/was pretty common practice to use dual points to jack up the dwell angle to something like 34° (a 75% duty cycle).