If you look at the periodic table then you can see calcium as a atomic weight of around 40. This is important because its a lot easier to figure out chemical maths when the unit of measure is on moles. To get from the usual parts per million (ppm) to micromoles, just divide the ppm figure by the atomic weight.
Therefore: 400 / 40 = 10mM. (NOTE: I'll round down from 40.08)
The other side of the issue is the alkalinity. When in balance the alkalinity as meq/l is
meq/l = (mM of Ca - 9) * 2. NB: The Red Sea has a difference balance, so rather than 9 we need to use 11
Note, once you go below NSW levels then the calculations go screwy.
By way of an example, let's say I have an alkalinity level of 3.5meq/l, what is the balanced Ca level.
(x - 9) * 2 = 3.5
(x - 9) = 1.75
x = 10.75 mM which in ppm
10.75 * 40 = 430 ppm
Turning it the other way round, let's say I have a Ca level of 450, what is the "balanced" alkalinity level.
450 / 40 = 11.25 (turn into millimoles)
(11.25 - 9) * 2 = 4.5 meq/l.