Ken. Factory preset is 80. Have you tried just using that? You should be able to hit a quarter deeper than 6" but you have to be in an area that has deep coins. I live in Denver and swing the atp. First make sure you have a spot that is clear of targets and then lower sens to two or three bars, ground balance, then lower the number by 10, manually.
Run the sens back up to one bar less than max. I run around 30 on iron descrim.
Last advice is to SLOW DOWN. Yes, the atp hits hard on targets 1,2,3,4 inches while swinging at high speed. Not so much with deeper targets. If you get a very marginal signal, slow waaaay down and go over it a few more times, really slow. Look at the depth meter...if it is showing real deep and it is a coin sized signal, try moving to standard coins and if you can get it to bell tone, dig it. That is the only time I use standard mode.
I used this method at Sloan's lake park except I had manually moved the ground balance number down by 15. At 15, the pinpoint seems to go away but a real deep coin os only going to hit in the middle of the coil so no pin point really needed. I do find that if I only lower the number by 5 or 10, the pinpoint still works and I am getting deeper. The coin I found at Sloan's was a 1920d merc at about 9 inches using stock coil. I was swinging very slow and got a squek high tone. Swinging back over it, just a small iron grunt, swing again and again, very slow, mostly getting just the grunt but the target seemed coin sized and I could get it to bell tone in standard coin so I dug it and was happy to see silver.
Deep coins takes patients and practice. Go to a place you know deep coins have been found and....HH ALL!