I feel like the code is missing some important bits. Like how FFT_N is incorporated into everything else, or where fft_log_out comes from.
You want to define your FFT information into 7 Buckets. Divide your output from your FFT evenly between the buckets, or center them around important frequencies, etc, and then just either average or sum up your values, and interpret that as your output. With 256 entries, it won't divide evenly, but you should be able to divide it up evenly within +/- 1 entry.