Hello again shutzen,
Let me be the first to say that I am no expert with any model of detector. And, I have only had my Classic IDX for a short time but I may be able to help some.
What I am about to tell you will not give you absolutely accurate measurements. It will give you some reasonable estimates.
Turn on your Classic ID and set the SENS as high as possible for the site. That is as high as it will go and the machine still operate smoothly. Find a patch of ground clear of all targets even trash targets. Set a coin on the ground in this clear area. Sweep the coil over the coin progressively higher and higher until the Classic no longer IDs the coin. Then while still sweeping, lower the coil just slightly till the ID returns. The distance from the ground that the coil is now at is the approximate depth the Classic can ID a coin of that type in this specific soil and at these settings. It is not perfectly accurate but it is close. Do this test with all coin types .... penny, nickle, etc. Two stacked quarters works pretty well as a substitute half dollar.
Now that you know the maximum depth your Classic will ID specific coins, you can use that information to estimate the depth of a detected coin. When you get a good solid ID on specific type of coin, continue sweeping over the target while starting to raise the coil. At the point you loose your ID, make note of how far off the ground your coil is. Subtract this distance from the maximum depth your Classic can ID a coin of this type. What remains is the approximate depth of the coin.
I am not a good writer so I hope the above made sense. It has worked for me. Again, if the math works out to an 8" depth, you know not to stop digging at 6".
Only one other suggestion. A good pinpointer really helps sometimes.
Good hunting
Hunt4Fun
I have a whites classic ID that I purchased as a backup unit and have a question about its operation. Is there any way to judge the depth of a target?