I've had a wide range on my AT Pro, and find that the solidness/crispness of the tone is more indicative of a good target than the VDI numbers. I've found gold rings at 44, well under the nickel reading, and also as high as 67, well above the nickel range. Also, everything in between.
When I hit a solid mid tone, I rapidly swing the coil over it from several directions. I can get the VDI for pull tabs, foil, and even nickels to bounce around a bit, but gold rings stay constant, maybe shifting 1-2 on the VDI, if at all.