I found a VERY interesting article, written by Randy Harley-Folmes:
I quote:
"implified Solubility Here's a good way for non-chemists to think of the solubility scenario occurring in typical reef tanks. First, let's discuss what happens in pure (fresh) water with dissolved calcium carbonate.
Each dissolved calcium ion is randomly floating around in the solution. Occasionally, it randomly bumps into a calcium carbonate surface. If it is a clean surface, it has a good chance of sticking. The higher the concentration of calcium, the more likely it is that ions are impinging onto the surface and sticking. This same dynamic holds true for carbonate ions.
At the same time, each calcium ion on the surface of the calcium carbonate can randomly come off of the crystal surface, and go into solution. Again, the same is true for carbonate ions.
After a balance has been established, the system has reached the exact level of saturation, with the overall number of ions coming off of the surface equaling the number going down onto it. This process is precisely how the solubility limit of any solid is determined. It is a surprise for many aquarists to learn that solubility is dynamic even at saturation, with ions dissolving off of the surface and precipitating onto it very rapidly, although in equal numbers.
Now consider a solution that is supersaturated. In pure (fresh) water, there will be more ions precipitating onto the surface than dissolving off of it, and the crystal grows. At the same time the concentration of ions in solution decreases. The crystal growth continues until the solid has taken enough ions from solution so that dissolution and precipitation are again balanced.
In reef tank water the situation is complicated by the phenomena described above involving magnesium, phosphate, and organics. In a sense, what happens is that the calcium ions (and carbonate ions) are less likely to stick to the surface when magnesium, phosphate, and organics are present. Consequently, the supersaturation can be maintained.
However, if W is too large, these phenomena can be overcome. Suppose a tiny portion of an aragonite crystal becomes "exposed" with no coating of magnesium, phosphate, or organics. When W is low enough, these covering ions will probably take over before much calcium carbonate can precipitate.
However, if W is too large, calcium and carbonate will "rain" down on the growing surface faster than the inhibiting molecules, and precipitation can continue until something stops it. In a runaway precipitation event, it may not be stopped until the calcium and alkalinity levels have declined to more normal levels, the "rain" has declined to a "drizzle", and the inhibiting ions can again cover the crystal surface."
From:
http://reefkeeping.com/issues/2002-04/rhf/feature/
Basically - what I read into this, is that EVERY system has a unique water chemical balance, and this chemical balance is reached at VERY different chemical levels, in different tanks. So - for one to say that you HAVE to have a calcium level of 380-475, would not be 100% true, as NO ONE knows exactly where your system's super-saturation levels are - the levels, which IF your chemical levels go above those levels - you could possibly experience precipitation.