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"I feel like there are so many iffy signals and 99% of the ones I dig are just junk ..."
My experience with the CTX (and the E-Trac) in iron strewn ground, if the Co-value is in a good region, though it may be jumpy but is jumpy in the good region, then dig. The Fe-value can dive into the high Fe-values with each sweep, but that should not be the deciding factor. You can think of the Co-value as the 'character' of the non-ferrous target, if the character holds close within a region then there is a high reliability that the target is exactly that, non-ferrous. The Fe-value simply indicates the amount of iron (nails) or iron minerals present but independently it cannot confirm the non-ferrous character of a target. Rather it is a clue to the ferrous nature of the ground matrix, which isn't the kind of info you need to decide whether to dig or not.
In other words, a high Fe-values combined with a Co-value that jumps about with each sweep is an iron target, but a high Fe-value combined with a somewhat stable Co-value is a non-ferrous target in a sea of iron. Don't expect the Co-value on a deep coin to be accurate, it is likely to creep up the Co-scale toward higher values and in some cases to lower values - the important detail is the stability not the absolute value.
Else, if you demand the CTX provide a stable Fe-Co pairing, then you are limited to shallow targets (< 6" deep) in moderate mineralized/iron ground. That's fine, but getting the deeper non-ferrous using that rational will cause you to pass the high Fe target by; the Fe value isn't the discriminator and shouldn't be relied upon. And, sure large iron will mimic a good target by projecting a more stable Co-value, nothing is foolproof. But if you know that old coins are likely, and they are expected to be deep, then you can't use a stable Fe-Co TID as your criteria to dig (unless you're in low mineralized soils such as coral sands). In the presence of iron you have to change the tactic you use to evaluate the signal.
Johnnyanglo