Joe,
I'll only add one thing to Monte's excellent reply.
Do be careful of relying on TID readings in the highest segments. Large iron can sometimes read as half/$$ and this causes many people to ignore those readings after digging some of the big iron. When hunting in heavy iron where pinpointing is impossible due to targets in close proximity to each other, I do not attempt to "size" targets either. Instead, analyze the target from different directions and if you are able get a halfway consistent lower reading go for it. The detector is not always just reading large iron as high coin, many times it will be a high conductor near iron "pushing" the reading up to the highest TID segment.
Tom