You always need to use a PRS (pressure regulating model) spray body for the MP series nozzles. Hunter makes a body specifically for the MP nozzles regulating the pressure at 40psi MPR40
Look for the grey cap on top.
If you want to maximize your radius reduction, you need to put it on a body with 30 PSI regulation (ex. RB 1800 series with PRS)
These comments are very helpful thanks - I wish I had known this before. I bought the rotators at my local irrigation shop in Sydney and they supplied the bodies without any comment to me about there being different pressures. I guess I just got the 40 psi model by default, as you say.
But I solved my problems by experimentation.
The way I figured it out was to leave the rotator in the "wrong" location, put buckets over the other rotators so they didn't interfere, and then let the side strip rotator spray onto my concrete drive. I used surveyors chalk to mark out the spray area for different adjustment settings (easy to do on the drive, but would have been difficult on the grass). That enabled me to see that:
(1) Ring adjustment: If I turn the ring to the left, the side of the area is straight (i.e. the area is a rectangle). If I turn it fully to the right, the side of the area is curved and the area is like a wide flat U shape with the rotator at the bottom of the U. The Hunter literature hints at this but doesn't say so. So I needed the ring turned to the left.
(2) Location: Since I was working at minimum 'radius' I needed to move the rotator 0.5m further away from the drive. (Of course, had I known, I could have got the 30 psi body, but my solution works OK).
So it seems to be AOK now, but I think I'll email Hunter to suggest better documentation for the side strip and for the body pressure. Thanks for your help and the explanations, which helped me understand the whole deal.