On my BH 505, it is very rare I bother with the depth indicator (if it did not have one, I would not miss it either) and as a rule of thumb, most coins will be found on the surface to at or about 4 inches deep.
You will love that 4" coil in trashed out areas, getting in closer to metal posts or fences and sheds, finding smaller items, and searching in soils with more mineral content.
Air testing can be helpful in learning a machines various responses to metals but the real education begins when hunting Old Mother Earth. And any of these machines with all the Bells and Whistles can be fooled because they are pretty much set up for the most sought after metal alloy contents that make up coins for instance and soils that would be most encountered. But if a piece of metal has about the same alloy make up as a coin, don't be surprised to dig up some foils when the machine said it was a silver coin. Not all bottle caps or pull tabs detect or display the same either. All my detectors but the BH 505 are hunted by sound only, no identifying tones or depth, but I keep a small collection of various things I've found through time that did not Tone Identify or Discriminate correctly. I keep a small Tackle Box with my smaller metal detecting stuff in it. Even for my Gold Machines, I have Hot Rocks I've saved and even made up a zip-lock bag full of black sand for my testing.
The best thing you can do is make a Test Garden and map it but first making sure there are no other metallic objects in it. And to make it interesting, put a nickle under a Hot rock or place a pull tab by a copper penny. In the real world of detecting, not every coin is going to be found free of other things around it or from being under the coil at the same time. Set up a garden that will drive it almost nuts.