Here's a link to Brian Felice's web page explaining all this:

http://members.cox.net/bdfelice/Ackerman/ackerman.htm

Best darned explanation on pull-pull I've come across. I agree with Mike - I only cross 'em when I need to, ans I go out of my way to avoid creating "ackerman". I make the distance across at the rudder post and servo arm the same to avoid binding... and I ensure the rudder horns are situated with the clevis holes on the control surface hinge line. This avoids complication and makes for smooth trouble free operation.