Thank you, Ferdman, for straightening me out. It's like a breath of fresh air when someone cares enough to respond.

With your response in hand, I went back and kept working with the distributor until I finally got it seated so that the rotor remained fixed. There is still some "play" in the rotor, meaning that I can move the rotor back and forth a little, but I'm assuming this is normal.

I think I've got the rotor positioned correctly but I'm still puzzled over how to check the 90 degree angle with the mark on the distributor rim that's mentioned in the "Mercedes Service CD".

Many thanks.