The term "dwell" literally means "the time the points 'dwell' together"(points closed). On the Chevy sixes, the dwell specifications are all over the map, anywhere from 32-33 degrees up to 39-42, depending on the year and distributor part number. I'd need to know your part number to find an accurate specification. Obviously, the design of the distributor cam has a lot to do with the necessary settings. The 66% rule of thumb is pretty much industry standard for the maximum coil magnetic saturation that still gives the condenser time to discharge properly between "points open" and "points close" cycles. If the condenser does not discharge properly, the arc across the points as they close leads to rapid metal transfer and point failure. There's a lot of physics and electronics involved in proper distributor design and function, and a lot fewer of us around who were educated on that subject since electronic ignition has been standard since the mid-1970's!
Jerry
