Kimmo Unfortunately, it’s mostly nonsense. A couple of good points but the focus on TDS and hardness affecting extraction has no data to back it up. Hardness makes the water taste better and affects flavour perception, but it doesn’t alter extraction - there is science to somewhat prove this but it is still something people quote, seemingly with no evidence other than inconsistent blind taste test results to back the idea up (inconsistent as in one test will show calcium is more important, another magnesium, and another that pure distilled is best).
On another note there’s no reason to decrease Calcium and replace with Magnesium, unless you want to perceive a little less body in the brew; the effect of magnesium is debatable but I’d say coffee seems a little sweeter with it. In terms of scaling potential, you’ll still get scale with Magnesium. It looks like you’ll be getting some scale in the service boiler of an espresso machine with your water…….but that’s easily manageable. If you want to alter anything I’d look at decreasing alkalinity (and pH) but this is personal preference and depends largely on the coffee too.