Still not exactly sure what you mean. Let’s see if I understand 
I’m hiding the first fft bucket which should have the representation of the DC and close to DC components of the signal.
The actual “virtual zero” of the samples is not relevant for the FFT calculation, and I’m actually using all positive (8-bit) samples from the ADC bitshfited (<<3) into a 16bit signed int, which is what is used in the FFT input/output array. (further bitshifting is done in the fft code to avoid overflows).
I also got code working for doing the fft calculations using 8-bit, but the signal-to-noise ratio seems to be too low for good results, although it works. Can’t get a decent scaling working with that.