Bowie,
I'm a bit rusty on the Ace 250 but it was my every day machine 5 or 6 years ago....if you run it too hot it has a tendency to false on you...I used to run the sensitivity between 4 and 6 and I managed to pick up 6 or so silver dimes on edge in powder dry conditions at 6" in depth.
You may want to go slow and do the overlaps to get the best possible coverage and not miss the targets....no a bad idea also to put ties or clips on your coil wire to keep it from flopping around....another reason which might cause falsing.
Hope this helps.
Regards + HH
Bill