It's not too important to set the dwell to an absolute point. Dwell is the period in rotation (per lobe, but averaged across four) the coil is allowed to charge. At low RPMs the rotor turns slower, and so the coil has more time to charge. It is only as the revs get to a very high point that the risk of the dwell charge angle not being enough becomes a problem. This will show itself as misfire. No misfire, no problem. Set the dwell too high however, the coil will simply overheat as once the coil field is charged it burns the rest off as a resistance.
So that is why the range is so wide. It's the range between high RPM misfire and low RPM overheating.
I do enjoy that the modern modules set the dwell intrinsically, I never liked fiddling with feeler gauges, I was terrible at it