Autorotational RPM is based on gross weight and DA, an increase in either one will increase the RRPM during autorotation, and a decrease in either will of course decrease the A-RRPM. the aircraft "should" be set as the MX manual indicates. generally for smaller helicopters it would be the average DA for the area the helicopter operates(seasonal) and the weight would be dictated by the MX manual. the mx manual should also have an equation for how much increase or decrease is expected per 1000 feet of DA or 100 pounds GW for example... 1% RRPM per 1000Ft DA, or 100 GW. so a typical mx pilot would take the aircraft to the test flight area and enter autorotation and record RRPM. after landing he could then decide what the RRPM should be. if the baseline DA is sealevel and the grossweight is 4500 pounds, and the helicopter was actually at 2000 DA and 5000 pounds when the test pilot took his readings. it would be expected to see +5 RRPM for GW and +2 for DA. so the RPM on the mx check should have read 107% assuming 100% is the optimum. this would be the correct RPM and any other RPM would have required adjustment. so now if the average line pilot goes out and does an autorotation at 4000 DA and 4000 pounds GW he would see a +4 and a -4 equaling 100% with collective full down. even though he expected the RPM to be high due to the 4000ft DA, the Gross weight reduction pulled the RPM back down. wow that was a long post. hope it helps clarify. and disclaimer... this info only applies to one design as each will be mx manual dependent!