I think depth is either sometimes exaggerated or some folks arent very good at guessing depth. Here in Ohio I have found very few old coins over 6 inches but it just depends on where you are at. I hunt an old school where wheats and silver are 6 or more inches, its nothing to dig a wheat or Indian at 8 inches. I have hunted a farm this year where I have found several silvers including a seated and 1825 L.C. and the L.C. was the deepest at 6 inches, the seated was maybe 4 inches and the 1864 Indian head was about 3 inches.
In my opinion, at least here in my part of Ohio I dont think coins are much deeper than say 6 inches on average but I have hit a few deeper ones and the E-trac had no problem finding them in a stock program running auto sensitivity at least up to 8 inches.
Indians - here in Ohio running in basically the stock coin program, I find the FE number is pretty erratic on most of my indians, it might go from 12 to 1 to 2,3... 5, 6 BUT my conductive number will stay consistant and even at depth they sound good so I dont worry about the FE numbers to much but USUALLY if my FE number stays on 1 and doesnt jump it turns out to be junk. The FE number will usually jump back to 12 or at least off of 1.
When hunting in discriminate mode and you come across a questionable signal that your not sure if it is silver or deep iron, hit your quick mask and dont have anything discriminated out, have the whole screen opened up. If your crosshairs jump in the bottom right hand corner it is iron... If it jumps from midway down the screen back up to top right hand corner it is likely a coin BUT I have found many coins at good depth including a 8 inch barber dime that the FE only jumped a few numbers, usually the FE will stay very close to 12.
I hope this helps, I am no expert but have found this to help me, hope it helps you!