Joe is right, the numbers vary some from who you talk to. It is just another way to help you decide.
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. A signal strength of the ground as follows 0 to 10% weak, 10% to 20% Moderate and 20% and up to be strong. Try to keep it at 20% or less. 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).