The DTI III is by far my favorite meter and very easy to calibrate for me as I check it often which a person should do.
The one way I use is to turn on the detector and run it in disc and either use a new dime or even a copper penny, run it across the coil and , or coil over the coin and try to get it to lock on 179-180 as it can vary just a little, by the control on the back of the meter. Just remember it dont take much to make a big difference. in the numbers. I try to get it to 179-180 and not 181 as it is too much for me. when I get it to those numbers I am ready to hunt with it and every once in a while I will check it again and throw down and new dime and scan it to see I get the 180 as i then know it is still calibrated. The knob on the back for calibrating can get bumped while transporting and can get out of calibration and when I get what sounds like a good signal and the meter read lower or higher than it should I will check my calibration.
the meter is sensitive and is very accurate when it is correctly calibrated . For me when I get a 176-177 it is a new zinc coated penny, IH or even some of the early wheat pennies, a 179-180 is a copper penny, dime of quarter while a 143-145 is a nickle and a 140-141 seem to be a beaver tail off the old pulltabs. normally a 145- 150 and sound like a nickle as they do have a tone a little different than a pull tab they do seem to be a war nickle..
Good luck as you have one of the best detector with the best meter when you get to know the GT
Rick