Good points by all. Lando beat me to it by suggesting another test for Ca
using a different kit. While the Salifert tests are pricey, you really can't beat them for accuracy. While I normally use API for my everyday tests, when I get a screwball result that doesn't make sense I have some Salifert kits that I pull out of the cupboard for a "sanity check." They usually confirm the fact that I might have missed counting a drop or vice versa, or gotten "heavy" drops!
Also... knowing what your source saltwater mixes up at, as Lando also pointed out, is crucial to knowing how to go about correcting any chemistry imbalance. If your source SW
mixes up at 360 Ca
(like my Reef Crystals does) and you want to maintain a Ca
level around 400, you know you're going to have to dose - PWCs just ain't gonna do it.
The other point made by Tangster that bears repeating is "don't dose unless you can test for it." And that means both Ca
AND alkalinity when you do a two-part Ca
solution. In fact, the back of the B-ionic bottle mentions that when starting to use the product to test DAILY and adjust dosage accordingly.