He doesn't use the coral pro - he uses the red sea regular salt. That mixes to around 7-8 dkH (depending on salinity).
I think switching salts in an established system is one of the worst things you can do. Like everyone says, stability is the key. Switching salts or changing your goals for the Alk, Ca and Mg creates a changing system. Exactly what you dont want.
I have no hard and fast evidence or studies to back this up - just my own experience. I got sucked into the marketing gimmick of red sea and their "total program." Soon after switching (from isntant ocean) - i started having nothing but problems in my tank until the eventual crash. There's any number of other possible reasons for my crash - and I dont attribute it the salt change alone. I count it as a contributing factor.
The main thing you really need to look at is what caused your imbalance in the first place? What was the event that drove your Mg so low? I'm thinking its not just coral growth that caused it. I'm thinking you've tried two part solutions to try to dose your system back into balance.
Think of it this way - all salts mix up to the randy holmes farley balanced graph (or it should and if it doesn't then it was probably a bad choice). So what happened that made one of the elements fall out of balance? If your high in Alk - then the Ca and Mg should also mix high.
Dosing to correct the problem is not the correct solution either. Dosing single elements like Ca, Alk and Mg is not "saltwater." Yea - it may contain Na, Ca, Mg, CO3, Cl, SO4, etc - which are the major components of saltwater - but if you have to overdose Ca to chase low calcium numbers - then you are also driving up the Cl concentration (without the corresponding addition of Na to keep it in the correct balance).
The only way I know of to fix the imbalance is massive water changes with the correct salinity so you change out near 100% of your water in a short period of time. This was a limiting factor in my system because of my saltwater mixing methods. It just couldn't be done with my set-up and manual production of ro/di and manual mixing.
Your goal should be to have the saltwater in your tank exactly match the parameters of your mixed up saltwater without using dosing between water changes as a means to correct inbalances.