My design is 4096-point FFT, which splits into 4 stages of radix-8 FFT.
The system twiddle factor is 15-bits and gives acceptable quantization
error.
However the design is bogged down by the trivial twiddle factor within
radix-8,
W81 = 2 * pi * 1/8 = 0.707, which can be reasonably done with 181/256.

However the quantization of 181/256 causes system quantization error from
80dB
to 30+dB, and this error absorbed quantization error variations with system
twiddle
factors varying between 13~20bits.

Consideration is the system memory (4096 point, 13b) won't be widdened to
accomodate this 0.707.

Are there any means to fix this problem?