There isn't a "perfect" level, and you'll do more harm to your tank trying to chase a perfect level than if you just find a range that works for your tank and try to maintain that.
All corals are different, all tanks are different, all flow levels are different, and all lighting levels are different. Different corals will use up minerals at different rates, different fish will do different thing to the water and the corals, different micro and macroorganisms will have different results... point is, chemistry in our water is a very complex beast, and it's impossible to give one uniform level for any given parameter that will always be the best.
Instead of shooting for a level, shoot for stability, and try and dial in your parameters to be about the same throughout most of the time.
Salinity: You can have success anywhere from 1.021 to 1.029 specific gravity. Best range seems to be clustered around 1.026.
Calcium: 400-480 seems to be best. Higher than that, and you can make your alkalinity volatile. Lower than that, and there isn't enough calcium for skeleton and shell formation.
Alkalinity: 7.0-11.0 dKh seems to be the range here, with stability being more important than any particular value. 8.0 is what a lot of the "systems" will tell you to shoot for, but that's more for it being a safe reference value than because of any particular magical impact at that level.
Magnesium: A range of 1200-1450 ppm seems to be the preferred level. Magnesium is a critical element for metabolism; too low, and there isn't enough. Too much can disrupt the metabolism of some corals.
Nitrates: We used to think that we wanted 0 nitrates, but now the research seems to show that low levels (5-10 ppm) are healthy for corals. Common refrain: Too little, and there isn't enough for corals to use. Too much, and it turns toxic.
So there are ranges to everything, and there really isn't a perfect level. That's to be expected; corals are long-lived animals that have been around for millions of years, and they need to have some ability to adapt to their environment to survive. However, that adaptability is tempered around the realities of life in tropical oceans. Water parameters in the open ocean tend to stay stable from minute to minute, hour to hour and day to day. In the ocean, you're looking at weeks to months for meaningful changes to happen, and so that's what corals are adapted for: slow, gradual changes taking place over a period of months.
