The only application where a nonmagnetic gauge is needed is in setting the air gap between the reluctor and pickup in an electronic distributor.

Dwell is the degrees of rotation the points are closed. The actual time varies with rpm. Set the gap, but the dwell is the final word for getting it right.

You need a dwell meter. Not so common these days but was the norm for any tune up work in the past. You may find a small digital one if they still sell them at the auto parts stores or you surely could find a nice older one on ebay.

If you are looking at spending any amount of money on a meter, I would just look at converting to electronic ignition instead. It will run better and never need points again. I like the stock mopar magnetic distributor, but if you just want to buy one new thing and your distributor is in good condition, the Pertronix kit is a very good way to go and simple. I have installed about a dozen of them and they all still work. Some of them are about 15 years old.

The dwell isn't a separate adjustment, per se. It is controlled by the point gap adjustment. Wider point gap = less dwell. Narrower point gap = more dwell. Checking the dwell with a dwellmeter gives you an idea of whether the distributor's point cam is in acceptable condition, or if it is worn. It also serves as a means of determining if a used set of breaker points is still in good enough condition to keep running. If the points' rubbing block has worn significantly, you won't be able to attain the correct point gap and the correct dwell at the same time.

So yes, a dwellmeter is a nice thing to have when you're tuning a points car, but you don't need one, strictly speaking, if the points distributor isn't going to stay in the car permanently and you're just trying to get it running. Set the point gap to the middle of the range, check it on all six point cam high spots, and move on.