The correct rotation of a rotating mast will give a much more efficient and powerful mainsail. It is thus important that the rig not be set up too tight as this can prevent full rotation.
There are many opinions on what the correct amount of rotation should be but a general guideline is to keep the mast rotated enough to give a smooth, even, transition from the mast to the mainsail on the leeward side.
Achieving the right amount of rotation will improve your speed and mainsail shape by pulling the apparent breeze through the slot efficiently. The amount of rotation will vary between 35° to 90° degrees.
Avoid allowing the mast to rotate or swing back and forth, which can happen in light winds combined with waves or when sailing off the wind with less sail pressure. This is usually prevented by having the mainsheet angled forward from the boom, which forces the boom forward to keep the mast rotated when pulled tight.
The amount of mast rotation is controlled by a line to the rotation arm on the mast from either the boom or the deck. This acts as more of a preventer than a positive control. The mast will naturally rotate in s position generally in line with the apparent breeze. The control line will prevent the mast from over rotating. A line from the boom has the advantage of being self-tacking by maintaining the mast at a constant rotation angle relative to the boom on all points of sail. However, the control line will have to be detached from a roller furling boom and transferred to an eye on the deck when furling the main.
A control line from the deck is also self-tacking, but it does not automatically adjust for different angles of sail. It will thus need to be let out when bearing off. Some racers like to fit two lines to the deck, one from each side for absolute control, but this can also be just another complication and is not necessary for general sailing.