Need help with BRS 2-part

[Reefgoat]

I recently realized my Alkalinity and Calcium are far too low so I bought a pre-portioned 2 part Calcium & Alkalinity package. In it there are 3 sets of packages for making 1 gallon each of Calcium and Alkalinity along with a package of Magnesium that says it makes 20 ounces and a large additional bag of Magnesium that says it makes 1 gallon. There is no container for the 20oz packs, only for the 1 gallon bag of Magnesium.

From what I understand I add the Calcium and Alkalinity in equal parts until both one gallon jugs are used up then add all the 20oz magnesium package at once. The instruction don't even mention the giant bag of magnesium that says it makes 1 gallon. Even adding the whole 20oz of magnesium in the package all at once seems like a terrible idea. Would someone please clarify the proper way to use the two separate packages of Magnesium.

This is for a 40g mixed reef. I read and re-read the instructions and watched the video BRS has online but they didn't answer my questions.