You were right, acceptable calcium levels are anywhere from 400 to 500. There really is no need to dose calcium if your level is at 400. You would really only need to dose if the stability wavered. As long as it stays at 400, you are fine.
Dosing calcium is trickier than dosing other supplements. You can't just mix the calcium and dump it in, it has to be dripped steadily as a topoff solution. Doing otherwise is a waste of time and money, and can even lead to consequences in your water quality.
If you plan on dosing calcium, you will also need to test your ph and alkalinity and keep them at stable, acceptable levels.