Lots of good advice here....
If I were you,, get the 5 x 8" dd coil put it on and use it the rest of the year,, a smaller coil helps you to learn any unit...
Hunt with a friend and compare targets,, guess at what it is,, dig the target,, and see who was right....
My hunting buddy uses a E-Trac and has for many years....He uses the 11" dd pro coil most of the time and I use the 5 x 8" coil...
Some targets are to deep for me to get an idea of what it is,,, but you will learn what a deep target sounds like,, it will be a faint sound,
to me it sounds like it is trying to climb in sound,, there won't be any number's to see....
Everyone has their own settings they like,, mine are
Pro mode,, zero disc.
Sens at max or as high as I can run where I am hunting..
Iron Disc at 35
Iron Audio on, always,,, that's the way I learned it and I stick with it.
GB spot on,,, our GB number's here are 80 + or -.....
I lower the sens down 2 or 3 bars from max and GB,, and then raise the sens back..
If its to trashy to GB,, I just set it at 80 and hunt......
The Freq. shift helps a little,,, but there is one freq. that my friend's E-Trac runs in sometimes,
just drives my AT nuts.....
Now, you have made some good finds,, reminds me of years ago, they were doing dirt work at a old school, built in like 1953,,, but before that
it was our state fair grounds,, from 1907 to 1953,, we found all kinds of good stuff there,, one day, I counted like 35 hunters hunting... This was a once in a life time hunt.
Some hunts are good and some hunts are not so good,,, but it beats the best day at working, lol.
Now, for your F-75, I have one with boost,, there is so much to this unit that's not in the manual,, but it will hunt, just about any way you set it...
I have other units, but grab the AT Pro with 5x8 coil and have fun.....
My 2 favorite coils are the 5x8 and the 4.5" super sniper coil,, for the places that I hunt... I love to hunt in trash,, the more trash,, the more I like it...
My 4.5" super sniper coil found me a nice 14K gold ring with one big diamond and 8 small diamonds..
So, just take your time and have fun and learn your AT Pro.....
BJ