The ratio is just what is present in a wide number of aquatic plant species and then averaged.
I looked up about 30 species and compared their Ca/Mg ratios.
The GH booster is a reflection of that ratio according to Greg Watson.
So if you add about 2-3 GH once a week, you will always have plenty of GH, if your tap has more, no problem.
BBS: you are quite correct, it makes no difference. Some enjoy arguing that it does, when they lack the control to maintain the other nutrients and thus cannot rule out a confounding factor in their conclusions.
Every single study I've ever found suggest as long as huge extremes are not done, the ratio of Ca.Mg does not matter in any horticultural setting I am aware of.