My experience has been, for nails the CO will often be 39-42 range the FE will vary but it is often a solid 12-41. However as Bart said the audio tells the story.
At a homesite spot with a lot of nails, and old coppers that register the same numbers, if I get an ok audio response all the way around I'll still dig for the, just in case factor, if it is broken and doesn't go all the way around I'll usually let it go or go back at the end(nothing but nails yet)
The just in case ones usually end up being a larger clump of corroded nail giving off a better than normal response.
A good repeatable audio response is rarely wrong, and if you find trash on a dig from that keep checking, there is probably something else there.
Most of my homesites yield results at 2-7" on coins, and it's always solid repeatable, the possibility of deeper ones is what keeps me checking the just in cases.