The F70 may in some cases ground balance to the same number as the F75, but it may not.
The F75 has more processing going on,
different coils will ground balance differently too, you may swap coils and see how the numbes compare?
But, in the end I wouldn't concern myself with matching ground balance numbers between the two detectors.
Now, I would get concerned if everywhere you go the F70 ground balances at 90 like its doing the same thing everywhere you go.
Here is something I started doing to speed up ground balancing on my F70.
I turn on the detector and switch to bank 2,
Set up some low power settings like sensitivity at 20,
A low threshold like -5,
then discrimination like 5 (anything LOW)
then I ground balance in bank 2 then I flip back to bank one where I have the normal search settings.
Yes! the ground balance carries back from bank 2 to bank 1.
The reason I do this is because I don't need a lot of horsepower to cancel out ground effects!
I can use the lower settings to search for a clean spot in the ground, in discrimination mode,
then ground balance there, and flip back to bank 1.
Both banks will retain last used settings except for ground balance.
I wasn't using bank 2 for anything else.
This process reduces effects of interference during the ground balancing process, or it seemed to for me.
Mark