The dominator is $10^6$ means that 1 ppm is added. Assuming the length of the symbols is $N = 1024$ as I insert the SFO for each symbol alone, so the maximum added SFO will be 1024$1024$ multiplied by the difference between $x[N]$ and $x[N-1]$.
On the other hand, if we can normalize the added SFO by $N$ to have the maximum added SFO 1ppm multiplied by the difference between $x[N]$ and $x[N-1]$, the above equation will be: