Bev, do you know where the link is for that calculator for these levels?
Incidently....
What processes inhibit continued precipitation of CaCO3 onto a growing crystal? The main thing happening in normal seawater is likely the impact of magnesium. It does two critical things (discussed in detail in this linked article): 1. Magnesium holds onto carbonate ions and reduces their free concentration, thereby reducing the likelihood of precipitation onto calcium carbonate surfaces.
2. Magnesium gets onto the growing surface of the crystal, essentially poisoning it for further precipitation of calcium carbonate.