If your beach/water hunting then dig anything above iron. If your land hunting and are mainly after old coins most people avoid any low or mid conductors (foil, tin, tabs, etc) and only go after deep targets reading like 173 on up, but mainly at least 176 to 180. Even if I'm strictly coin hunting I still dig any solid number from about 139 to 148 because it could be an old nickle or even a gold ring, but most nickles will read somewhere between 142 and 146. The trick is they don't wander off in digits at all or at most by 1 or 2 digits, sometimes 3 but rarely. If you hit a target in that range sweep around it from several directions. If it sound fuzzy or keeps changing in VDI numbers then it's probably junk. Nickles, like gold rings and other coins, will give a solid hit, more "robust" tone, and not really change all that much in VDI unless very deep.