Open sights?
If that's the case zero 1" or so low at 25-30 yards. The flip ups sight are closer to the bore than the standard front sight tower. With a standard from sight tower and rear sight to match. A expedient 100 yard zero done at 25 yards would be 1.5 inches low but seeing the sights that come on a M400 are lower I would start with at 1" low and go from there.
That should put you on the Easter Bunnies head across a average living room.👍