Daver,
The aircraft maintenance manual should have a paragraph giving the parameters required to be observed and a graph to work them through. Different aircraft have different procedures, but the aim is still the same, the flight manual may also provide the procedures. If the auto revs are out of the limits it is normally a fairly simple adjustment by a licensed engineer (duplicate inspections will be required) followed by a re-check. It is better to carry out the check when the aircraft is light rather than heavy. Also make sure that your OAT indicator is correctly calibrated as well as other indicators. Most people will say that too many revs are better than too few, for obvious reasons.