From my experience Martin, which is very limited, I found that it's not necessarily how many darks and flats that you use, it's the ratio between them & your light subs. At ISO400, it's unlikely you're going to have enough signal noise to require so many darks. I do agree that in light polluted skies whether it be the moon or artificial, slightly more darks are helpful.You do however pay the price on the other end in dragging colour out in post processing. I'm sure you'll find many people to disagree but that's just my experience. Having said that, It's a fantastic shot for the start of your journey
