Complete beaver tail pull tabs hit in the 16 to 19 range in my bad dirt too. The most modern pull tabs will hit anywhere from 13 to 15 if they are not bent or damaged. The older square tabs hit 11 to 13 if they are not badly damaged.......
I use the pinpoint function a lot to help separate deeper targets. Also, if I can get repeatable numbers and tones with tiny sweeps in one direction on deep coin sized targets, I'm probably digging. If I am lucky enough to get relatively tight numbers and tone in two directions 90 degrees apart, I am definitely digging.
Having a really good, quiet, coil pumping ground balance setting has really helped me on deeper targets and also helps more accurately pinpoint..........
Jeff