It seems that no
matter how I attempt to adjust the reactor, I cannot get the calcium
level above 400. Can I assume that I am doing something wrong in
adjusting the reactor, or is calcium supplementation still required
even in tanks with a properly functioning calcium reactor. P.S. I
use ARM media from Carib-sea.

Alkalinity levels 11.5-12.5

There is nothing wrong with Ca at 400ppm as it is the natural balance level.
Higher levels will only get things out of whack. Your levels are fine and
there is no need to make any adjustments.

jc