Let the machine "warm up" for about five or six minutes or so before messing with the meter. I find that if I adjust it right at power up I have to re-calibrate several minutes later as the machine reaches what is called "steady state" in the HVAC field. Not going to hurt things to adjust it right off the bat, but it might be off a bit once the machine warms up, so check again after about ten minutes or so to see if things are still reading right. That's why I tell people to never mess with the meter after power up until it gets ran for a bit, as chances are it will read right when things warm up so long as you haven't bumped the calibration pot. It's not a good idea to keep tweaking with that pot all the time, as pots have a limited life based on the number of times they are adjusted. The more you play with them, the more they develop dead spots and thus make it harder and harder to calibrate.