I use an F70, I hunt some very trashy parks...always have.
Not so much a coin hunter although I do find them at some older sites, but what I do look for is jewelry is jewelry, especially gold, and I have been somewhat successful.
I can tell you that this up-down averaging effect is also true in hunting that elusive precious metal...and silver too.
I rarely disc out much higher than 15, most of the time I hover between 4 and 6.
In areas with a lot of small foil or can slaw I might go into the lower 20's, but since recently reading a post where someone found a gold chain at 18 I probably won't do that much anymore.
I NEVER use notch.
I hunt in trash several different ways, 2 of the best ways involve both disc and all metal.
Using disc I have found NasaTom's advice of disc 6, mono tone, I usually am in low negative thresh, DE speed for the fastest processing.
Sense usually from 60 on up, 85 is normal for me if it stays relatively quiet.
In really heavy trash I lower the sense to between 30 and 40.
These disc settings seem to give me the most stable info on good targets in heavy trash, even when using the big DD coil.
I look for some stable repeating numbers, even a flash of them, and check out targets from a couple different angles.
It works pretty well at plucking out good targets among trash.
I don't like discing out trash when I can, or notching out trash ever because there is too much I could miss that way.
I just get all signals and quickly examine each one and ignore the ones I can't get stable quickly.
What works very well for me, better really especially in heavy EMI, is full blast settings of 99 sense, 9 thresh, all metal, and on this particular target in the pic below I was in SL although nowadays I use DE more if major depth isn't my concern.
This gold target was nestled nicely between 2 pieces of rusty iron.
One was deeper an inch or two on the right, the kind that throws off some of those high tones, the other was an inch or two on the left shallow and smaller like an old nail.
Luckily I hit this thing from the right angle, from right to left across each different target...if I hit all three at the same time from 90degrees I have no idea what kind of signal I would have gotten.
As it was that big coil had to cover all 3 of these targets at some point, and those that swing Fishers must have a clue about what was going on with the jumping and the audio in this area that also had more EMI problems than most sites I have ever hunted before or since, but I kept seeing flashes of some stable numbers, 51-53 as I swung back and forth over the area.
That is always my cue to dig so I did and this 10k beauty popped up.
Out of the ground it dropped 10 numbers to a solid 41 with every pass so those iron objects definitely affected that signal and up- averaged it.
I can see the same effect happening in the field with coins and all kinds of other jewelry so I believe and stay on my toes at all times.