On inquiring some time at AHPC concerning this subject, I received the following reply:
"The mast rotation should be at about 80deg on the downwind leg. The tuning guide is not accurate in this description as 80 deg would have the mast spanner pointing slightly forward of the sidestay.
In practice if the mast is completely free to rotate downwind at times it will rotate past 90 deg. The only problem with this is that the boat is not quite as fast. If the mast is rotated a lot less, say 50 deg, then the mast bends too much sideways which can be bad for the mast. Also the air flow around the mast is not as good and the boat is slower."