There is no specific VDI for a $5 gold coin. If it is laying flat it will give one VDI. If it is not laying flat it will give a different VDI. Different soils will have an effect on the VDI as well as other factors.
What ever you do in this hobby, learn tones, not lying VDI's.
Don't need the exact number, but something close to it. I am trying to set a notch for gold rings and that is the upper limit. Trying to translate a "50" from a White's which is very near $5 gold coin.