I'm guessing you've done the dig everything option to death so I will try and help.
The table in the op to the 705 rings almost true to my id (f75) on my machine except my numbers are double the value.
Gram nuggets or bigger from here in vic come in just under the 5c and pulltabs, (modern pull tabs on the surface) but as my machine is set up internally to do coin size objects it has a little variation when you add in depth and swing orientation in relation to position and size of nugget.
So my id for 5c is 23, pulltabs at 21 and nuggets at 20-21 on the coil just above foil which runs from 17 to 19.
My guess is if your id is between foil and 5c pieces you are in the right range. How many 5c pieces or foil do you think are out there? My guess is bugger all, and in taking my thinking a step further if you notch out everything but the range you should be in the vicinity.
Maybe try a test nugget from near the area above a gram, bury it and experiment and you should be in with a chance I think that's about the only way to test just how capable and accurate it's likely to be.