I understand what you are saying bertoni. BTW, I'm not disagreeing either just that a "formula" can easily be "created" for each person's tank based on what you want the overall outcome to be (normal running levels). Just break everything down to 1 dHK (Alk) like I did in the example.
Let's say your a person fighting bryopsis and want to keep your tank at 12dKH Alk, 380 Ca and 1600Mg. You could divide each value by 12 and come up with:
1dKH Alk, 31.6 Ca, 133.3Mg
(again only an example but I wouldn't recommend using this)
I do not think one size fits all. For example, I'm not a fan of 10-11 dHK levels for alkalinity. I find overall color and health of my tanks at 7-8 dHK the best (also very close to NSW) but intentionally run at 8-9dHK for a little wiggle room. I don't ever see any problems unless the dHK gets below 7 (in my system) which I don't let happen. With a Ca reactor, Kalk drip and pH reactor all controlled via the Aquacontroller it stays pretty steady at about 8.5 for me. Again unless you have really good control over your alk you are probably better a little higher to give you more "room" to work.
Honestly, the only "real use" I see from keeping things in balance is when you are mixing up salt and need to deviate from your typical mix. Say for example your salinity is a little higher then normal due to evaporation and you want to mix up a little weaker salt mix and do a water change. Knowing how each part equates to each other can help you mix the salt right so the outcome is pretty much spot on after the water change. This can be useful for raising or lowering salinity via water changes or even topoff if on the low side. By keeping the mix water in balance with your tank assuming you don't get precip, or drastic changes to your tank when doing water changes.
I think this whole "topic" is a bit overboard for the average reefer if they keep their levels in relative check and use a decent salt mix. However, some salt mixes are cheap but so far out of balance that it makes sense to adjust the mixed salt to your levels prior to using the salt.
Another use of the "formula" could be used for dosing. For example, the 2 part solutions don't "balance" correctly for some people as the alkalinity gets used up quicker then the Calcium in their system. Since the 2 part ionic balance is based on calcification but there are other processes in the tank that can use up alkalinity. So if you understand your tank's ionic balance and uptake you can adjust your homemade 2/3 parts to suit your system better. But again even without understanding this you could just go by the levels tested and just keep the tank at an overal value of X (insert your number here).
Carlo