Calcium reactor maxed out?

I have a 2 1/2 year old 150 gallon reef tank containing a variety of
SPS, LPS and fish. I have been running a Korallin Kalkreaktor model
1502 calcium reactor for nearly two years. This model is advertised
to be appropriate for a tank up to 800 gallons, so I thought it should
be more than adequate for a 150 gallon tank. Quite frankly I have not
monitored the calcium reactor regularly until the past several months
when I have obtained a varitey of SPS and a clam which of course
benefit from higher Calcium levels in the tank. 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