Honestly, most all quality fertilizers have plenty enough Mg/Calcium in them to supplement your plant with all it needs to be happy. Most all Mg def comes from over-feeding, pH issues with the soil or grow medium, or lack of it in the fertilizer you are using.
The back label of the fertilizer you use should tell you if it has Mg in it or not. If it does, you are fine. Adding yet another additive won't give you the results you are hoping for(bigger buds). The ONLY thing that is going to get you bigger buds is healthy plants, and experience under your belt.
Epsom salts are not something you want to use every time you feed. You are asking for lockout of other nutrients if you get a buildup of Mg in your plants.
If you honestly want more Mg/Calcium/ in your grow then the easiest way, safest way, and the ORGANIC way is to add a Tbsp of Unsulphured Black Strap Molasses to your nutrient program.
Epsom salts is mainly used for a quick fix until the grower figures out the "real" problem. Which is normally one of the problems I stated,pH, Over feeding, ect.
If you want HUGE BUDS, here is the best way....you ready?
KEEP IT SIMPLE STUPID
The K.I.S.S. method really will get you better buds in the end.Couple that with Experience, great genetics, and a great grow environment and you will achieve everything possible with your setup.