If I'm not mistaken...phosguard is only suspose to be used for a two day max period then it says it will leach back into the system if not removed and new media put in. If you have kept it in the sump for more than a day or two, I'd say you might be getting some leaching back into your system. I was following the old post..so I'm gonna go reread it again..but usually when your mg is low you cannot raise Ca.
here is a chemistry calculator.....believe me it works...dose half what it recommends to take into account rocks and such in the tank....see where you stand at how much chemicals you have been dosing and what is recommended...I was never able to get my dosing schedule down until I used this....
Reef Chemistry Calculator