Not many good targets will CONSISTENTLY hit above 13 Ferrous, Rick, but many (deeper ones, and ones with trash around them) will hit there OCCASIONALLY, as you sweep over the target multiple times. I hunt almost exclusively in iron mask, set at 22 on my SE Pro (which is the equivalent of -6, I believe, on an Explorer II). While most good targets will NOT hit as high as the low 20s (or even upper teens), the fact that they can, on an occasionalal sweep, means that you have a better chance of getting that initial "chirp" on a target when you don't set your disc. pattern or iron mask too tight. You NEED to get that initial chirp -- the one that gets you to stop and investigate, but you DON'T want every piece of iron trash to chirp...so it's a matter of going "loose enough" on your disc. pattern (or "open enough" on your iron mask) so as to maximize your chances of hearing the "good chirps," but minimizing the number of "iron" hits you'll hear.
Yes, you COULD do what you are saying in Ferrous sounds, digging anything at 13 or below. You will dig alot of junk that way, but you won't miss a WHOLE lot, if you move slow enough. It's not a bad plan, IMO...as long as you can tolerate the abundant pieces of non-ferrous junk you'll dig...
I think though that a strategy as you have proposed, focusing on one of the numbers -- either the FE number OR the CO number -- is tossing out some good info. To me, the COMBINATION of the two numbers contains more info that just one, to the exclusion of the other. In other words, if you dig EVERY target with FE 13 or lower, you will dig multitudes of screw caps, pull tabs, rectangular tabs, etc. etc. etc. On the other hand, if you dig all CO numbers higher than, say, 24 or something, you will dig tons of iron falses, aluminum screw caps, etc. etc. Using BOTH numbers, and knowing roughly where "good" targets should hit, is the better bet IMO.
To me, a good approach to deep, old coin hunting should include these three things:
1. Decided on ferrous sounds OR conductive sounds, and then STAY there, so that you can train your ears in terms of what a "good" target sounds like, over time;
2. Dig only targets that show good repeatability/consistency, as you rotate 360 degrees around the target doing a constant "wiggle" comprised of very short sweeps over the center of the target; and
3. When deep coin hunting, dig targets with an AVERAGE conductivity number over 23 or 24, and an AVERAGE ferrous number lower than the mid teens. BOUNCES higher or lower, on either number, can occur, but in general, the lower the average FE value COMBINED with an average CO value above that 23 or 24 range, is the best bet IMO. (Realize though that doing this, you will miss all old nickels, and most gold coins...)
Steve
Steve