And `0.5 * log2(2.0 * M_PI * wdf_lower / len_upper)` == `0.5 * (log2(2.0 * M_PI * wdf_lower) - log2(len_upper))` == `0.5 * log2(2.0 * M_PI * wdf_lower) - 0.5 * log2(len_upper)` and then you can collect the two multiples of `log2(len_upper)` together...