As others have said, it can depend on the soil your digging, the condition of the coin itself, as well as the year the IH cent was minted (metallic composition) e.g. older "fatties" come in the lowest. The Target Separation mode also has an effect on the conductive number, to a lesser degree. As a whole, I've found IH cents anywhere from 12/22 to 12/36. Most zincs I've dug come in at 12/37. And again, as others have mentioned, corroded zincs just don't sound consistent.
Here is a link to a coin chart I have posted over on the CTX Resource Center.
http://www.findmall.com/read.php?87,1731336,1731336#msg-1731336