I thought I would chime in and answer this. I always check rotor phasing on a performance engine and it is a must if you use electronic timing control like the original poster.Why do you need one?
Just to get my head around it, if I understand correct there is quite an angle (rotation) that the rotor is before-at-after a cylinder tip where it can transfer the voltage correct?When you use a electronic timing advance or delay controlled from the ignition box or ECU it is electronically delaying or advancing when the ignition fires the cylinders. Because of this you can get the rotor being somewhat out of phase with the cap because nothing actually moves in the distributor to maintain rotor phasing.
Don't worry about that, now you said it I also realized I never gave that a thought.believing the rotor would be different in a Mopar MSD distributor than a Chevy. The caps and rotors are universal to the design/all the same. Just another brain fart in my old age I guess.
So in this time (in crank degrees) the rotor is at/near the post is where an ignition control can control the timing by advancing or retarding the coil to create the spark.Yes you are quite right - there is some tolerance either way either to the right or left of the post.
You cannot get it totally perfect just close enough that you do not have a misfire, a crossfire in the cap or waste spark energy jumping a super wide gap.
You try and adjust to a point were the rotor "sweeps" past the cap post through the timing curve.