I've said it before and I'll say it again...IN MY SOIL, which tends to be medium to high in mineralization (some spots aren't, though), something much lower than max stable sensitivity will give me best ID and audio at depth on many occasions. I found this to be true most of the time with the 15x12, and I think that was because it was soaking in more ground matrix. Making the coil too hot can null, degrade, or wash out the coin signal mixed in with it IMO. Now, with the stock 10" coil I also found this to be the case at many sites. With the 12x10, however, I'm finding that more often it wants to run at max stable sensitivity, even a little into unstability where the threshold will reset say every 10 or 20 seconds as you hunt. You do have to move slower, though, with high sensitivity. My main tool for determining where to set it is to bury a silver dime at say 6 or 7" deep, sometimes deeper. I like to get it right to the edge of detection for that site on a newly buried target like that (stuff in the ground a long time will ID much deeper) and then play with the sensitivity doing short sweeps or wiggles over the target. Many times around 1:30 to 2PM is the sweet spot where the ID/audio is best, but as said the 12x10 has me riding the edge of stability more often than not and that's usually 10, 11, or even full blast sensitivity as this coil can often run with it all the way up.
Go too high with sensitivity and, even though the machine is stable, the ID will range wildly and even null or sound bad. Often backing it off a good bit will give a more rock solid ID. Too high can make a good target look like oddly shaped junk. Don't worry so much about how deep the machine is getting if you back off sensitivity. I've dug coins 9.5" deep with sensitivity set at 2 or 3:30PM on land, and at least that deep and deeper in the sand. But as said the 12x10 wants to run maxed many times for me. Just make sure to move much slower and work the coil in slow motions over and around signals to try to pull out a high tone.