I haven't used extra CalMag so far, but I might just have been lucky because my tap water already contains a lot of calcium and magnesium.
It is recommended to give a CalMag supplement when using coco; or you can use hydroponic fertilizer that is made for coco growing, it already contains the supplement.
The Lucas formula has nothing to do with this. I honestly don't get the point of doing Lucas, you have to handle one fewer bottle of fertilizer, saving you maybe 2 minutes of your time, and that's it. Does not save you money. You're taking a three-part fertilizer and turn it into a two-part fertilizer. Why not buy a two-part fertilizer in the first place? There is a reason for it to be composed of three parts, there's rather complex chemistry involved, I wouldn't mess with the manufacturers formula unless I understand in detail what that entails.
If you use General Hydroponics fertilizer, either use their TriPart at half-strength and add CalMag, or use the two-part Coco fertilizer that they make (probably use that one at half-strength as well, haven't tried). The recommended full strength schedule is much too high for Cannabis, you will burn your plants; happened to me...
Pre-buffering the coco certainly isn't a bad idea; use a CalMag solution or a Coco fertilizer to pre-buffer it (by soaking it in nutrient solution, then drain before use).