A magneto rebuilt by a competent and trustworthy technician, should come back with a properly centered cam ring, but likely not a NEW cam ring, as they are rather expensive (about $100) and a rebuilder is likely to let anything passable thru. IF the ring IS centered, then both point gaps will be equal (the rebuilder should do this much), then IF the ramps on the ring have been machined 180 degrees apart, the magneto will fire the cylinders 360 degrees apart.
If the cam ring is machined spot on, that is, the ramps are exactly 180 deg apart AND the ramp thickness is exactly equal on each, but the ring is off center, then the point gaps WILL be unequal and the engine will fire early on the side with the larger gap. Thus it becomes imperative to check the point gaps at the very start.
With a new CNC machined cam ring, and point gaps within 0.001 inch, the timing on each cylinder to within 2 deg should be easily achieved. There is nothing inherent in sloppy timing except that old stock cam rings were sloppy, having been machined on war worn equipment. As Bernhard says, NOS cam rings often had ramps off 180 degree apart. The only remedy to fix these was to stone down the early ramp leading edge .... better to fit a new CNC ring.
Regarding getting off the lower nut: It is the most bitch of anything I have ever encountered in shade tree mechanics! Tips .... remove the outer gear box cover and the oil junction block, if only to see better. I ground some of the meat off a Whitworth spanner to get just a little more turn. Be sure the threads on the stud and nut are perfectly clean before replacing the nut so you can spin it on with your fingers, and do not use LockTite. NOTE: Later (1965+ ?) replaced the lower stud with a bolt, and the nut was accessible from inside the timing case. For those with such machines, these tips do not apply.
Once you get the nuts off, you can make the job easier the next time by getting nuts one size smaller, then drilling and tapping the smaller nuts to fit the studs.
Regarding self extracting nut: They work great when they work. When they don't, it is usually because they strip the internal left hand thread. Your method of a sharp rap with a hammer is first option if you can get some tension with the internal thread. If completely stripped, as mine is, then next option is a small gear puller.... back off the center bolt 1-2 turns, then pull on sprocket while pushing on center bolt.
Regarding timing set point.
When the 32 deg specification was written, 100 Octane fuel could be found at any pump. Moreover, the factory stock Atlas had only 7.5:1 compression ratio. With modern fuels, especially if the machine has been fitted with 9:1 or higher pistons, then the 28 degree setting that the Cdo guys target for is more appropriate. Mine is set to 30 degrees.
Finally, how much difference does 1 deg make? Not much, just like cutting one slice off a baloney ... one slice is hardly noticeable, but sooner or later you don't have any baloney. 
Seriously, one has to try and see how the machine runs ... CR and the fuel octane make a greater difference, as well as how you operate the machine.
Slick
