I don't agree . May be you mean there is no substantial chemical reason? I think one should target the natural ratio magnesium/calcium (and other major ions), and I cannot find any substantial reason to not use natural ratio which is relatively stable in world ocean for very long period of time and is quite independent of salinity (with only few exceptions).Actually, I don't agree at all. I think one should target the desirable range of magnesium (say, 1250-1400 ppm) independent of what the calcium level actually is.
There is no substantial reason that just because calcium might be on the high or low side to target a magnesium level that is high or low.