Ground Probe
Ground should fall between 0 and 180 degrees. In air, when you zero the detector, you will see phases of both positive and negative values - those are just noise and don't pay attention to them. If you are getting a negative phase for any of the frequencies when you lower the loop to the ground however, you probably have an overload problem. Most ground that we see is 178 (-94) to 165 degrees except salt which falls close to 90 degrees (VDI 0).
Nonferrous targets (coins and such).VDI 0 - +95
The VDI reading is converted from the ground phase normalized to standard VDI units. It's either going to be from the strongest signal if the detector is running best data, or the average of the two strongest signals if correlate is selected. If you have ground coming in at -94(VDI) with a signal strength of 2% and another ground coming in at -91 with a signal strength of 7%, it would seem to me that the ground coming in at -91 is more mineralized even though it is a less negative VDI. 0-10%=low mineralization, 11-25%=moderate and 25% on up = high/very high. The lower the mineralization, the slower one should sweep - which may mean a lower filter should be selected. The phase tells you composition (between purely ferrous and purely salt), while the signal strength tells you the concentration (more mineralized).