The GH component of your water is made up mostly of Ca and Mg. I'm not sure why it would be 'best' to add Mg. It appears that in more cases (mine, I believe), a high GH can be predominantly made up of Ca, so you can still be Mg deficient. So, I'd guess that the likelihood of Mg being low is higher, so start by adding Mg if you feel you have a deficiency. If you are looking to just raise the GH, because you want to, then you should likely add both, and there's some proportion folks will recommend...though I haven't really thought through why that should really make a difference.