Oh come on! That 42.5 ppm Mg level is just a quote from that source. There is no way that we can define it as 'optimum'. The other thing that you should take away from that article is that the malt contributes 100% of the Ca and Mg that the yeast need for their fermentation. Adding any more of those ions via the water is at the brewer's discretion and should be made based on their goals. For instance, if you wanted the beer to clear rapidly, then do add Ca. If you want more Cl or SO4, then the brewer might consider more of a salt that contains Ca or Mg to supply those anions for flavor.
I wouldn't worry too much about the Mg:Ca ratio, but it is important to recognize that you may not want to add a boatload of Ca to your brewing water and push that ratio into an unfavorable range. You should recognize that the additions of Mg and Ca to the water are typically going to be modest in comparison to the concentrations of those ions added by the malt. So as long as you aren't overdoing mineral additions in the water, your wort is probably going to be in an OK range.
Regarding starter preparation, yes it may be reasonable to add a Mg salt to the water. I suggested that in the article as a way to help infuse the yeast with a little extra Mg for the main ferment. However, I suggest that you might best prepare your yeast by generally mimicking the water profile that you will be brewing that next batch with. I wouldn't worry too much about it though.
Let me correct your take home message! Ca additions for the mash ARE beneficial since they help precipitate oxalate from the wort. I suggest that all brewers always include at least 40 ppm Ca in the mash for that purpose. But that does not mean that your overall Ca content in the kettle needs to be that high. For my recent lagers, I've been adding Ca salts to the mash and none to the sparging water in order to end up with low Ca content in the kettle. So target at least 40 ppm Ca in the mash and whatever you want in the kettle (may be higher or lower). By the way, the latest supporter's version of Bru'n Water includes a setting so that you can tell the program that you want to add all the sparging minerals to the mash to create the technique I mention above. The program also reports the ion levels in the mash and kettle separately so that you can assess that you are boosting that Ca level to that desirable 40+ ppm level.