You will get a really good feeling for it if you run your detector in the same settings all the time (i.e. you will get better and better at being within an inch of the depth "guess".
Drop a quarter on the ground with Disc 1 and Sensitivity set where you use them the most often. Pass coil over quarter. Raise coil 10 inches off the ground. Somewhere around 10 inches you should here the signal dull a bit. Measure the height over the quarter where you start discerning that difference. Repeat with penny, dime, nickel, quarter, and wedding band. Figure out the average height at which the signal dulls.
Example at 3 Disc/ 10 Sensitivity:
Quarter 9"
Dime 7.5"
Nickel 7.5"
Penny 7"
Ring 6"
Average = ~7.5"
Scan the lawn for some buried metal. When you have a signal, raise the coil off the ground 6" and scan again. If you still get a clear beep, raise the coil a few more inches and scan again. If you are still getting a clear signal, leave your digger in your pocket and scratch around the surface where it is pinpointed as the target is pretty near the surface. Don't use too much force with your finger as you don't want to get jabbed by something.
If you raise the coil only an inch or two off the ground from your normal sweep height and the signal is softening, figure you will be digging at least 6" down.
You will find your time on your knees digging decreases A LOT if you can get good at it.
Matt