All grills will have different temp zones so you will need to learn what yours are. What I did (on my Weber kettle) was place a probe at the grates, just as SmokinAl suggested, and I watched the dome therm and the probe as temps went up. I was able to deduce that the dome temp was about 5 deg F hotter than at the grates so now I just calibrate against that and use the dome therm.
Also note that this temp difference is based on the particular therm that is at my dome. If I were to change that out, the difference would probably change. the bottom line is that you want to get your true temp at the grates using a reliable therm and then you can calibrate any other therm you have located elsewhere to determine the grate temps.