Lug nut torque

As a Snap On Dealer I sold lots of them. They are as good as the user. They are rated to be used at a 225 ft lb. output on an impact gun. So you have to set your impact gun for example level 1 or 2 then verify with a torque wrench. If not tight enough try level 3 and so on. The problem also lies with the compressor. Did it just kick in or just kick out as that also makes a difference. If you are carrying a small portable for adding air to a tire with a 1 gallon reserve it would be totally out of question. As I mentioned I read the instructions and sold lots of them took the time to explain procedure but can't say I ever noticed anyone ever listened or cared. They only wanted the service manager seeing them using a torque stick. At the time I retired I sold guns putting out 800 ft. lbs. of torque and some used 1 torque stick for any application and in most cases were severely over torquing.
Hope this answers the question of how accurate can they be.