Okay, here's my thoughts. On etrac I stuck a couple nice silver all the way down to 27fe reading. They were both over 9 inches deep in an iron patch using a 10x12 sef coil. On the CTX the farthest down I've locked on a good target was a reading of 24ferrous a musket ball down deep and I was operating in ferrous coin target separation. I feel like 25 is far enough on the CTX while running in ferrous coin target separation. In the other target separation modes I would still opt for the 27 ferrous line in setting up my disc. Remember your ground will be a deciding factor. In sweet ground you will probably never ever see a good target show over 22 ferrous with the CTX in ferrous coin target separation. But also remember a colocated coin with iron could drive the ferrrous number up. There is never a for sure thing. I think I've read some say 25 is a good cutoff point for the CTX. It's your decision.