Brad, if it's M. C. Hexahydrate, you'll want to mix 5cups of it with 3cups Epsom salts into RO and after it's mostly dissolved top it up to 1 US gallon/3785ml
This will last you quite awhile, once your Mag is up where you want it
For dosing go to this
Reef Chemistry Calculator and select Randy's Recipes 1 and 2 versions A and B
Don't miss the top window where you add your system volume if you want the calculator to actually work
Even though you have a Calcium Reactor, info on the Recipes is here;
An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System
It'll tell you more about the Mag portion the Recipe
Edit: I didn't realise Randy wrote a separate article just for Mag
Thanks for the link