The standard numbers are something like - Fe= at least 0.5 ppm, Mg= more or less 10ppm, Ca= no standard really but for sure at least 2ppm. Again, none of these numbers are cast in concrete as individual plants may do poorly unless the numbers are higher or may do well with lower numbers. Mg and Ca in particular are difficult to pin down but 2ppm for either would be a bare minimum. If you use normal tap water which is harder than 1 or 2 GH you probably have enough Ca, but even with GH10 or higher JamesC proved that you may not have enough Mg. Dosing Epsom salts cures any Mg shortage. Fe is a lot easier because it is usually the highest component of any trace mix.
I hope you are not planning on buying test kits for these?
Cocnetration over time is really the question, especially for traces and some other typically limiting nutrients.
Example, rather than some static residual level you "maintain", you have a pulse of say 0.4 ppm of Fe that last say 2 hours before hitting 0.0ppm.
Now that may or may not be enough for the plants at a given growth rate.
But the test kit will not really tell you much either.
And it would depend on how soon afterwards you tested.
Say you added 4.0ppm now, and the next day(24 hours later, you had 0.2ppm.
You added 10X as much,m but at many rates of growth folks have in their tanks, the original dose at 10X less was fine.
Some things do not last long enough to use test kits unless you plan on testing each hour, 15 min etc after you dose.
Relating that to plant health is extremely tough.
So how to get around this?
Simple, you over dose and maintain the other nutrients high as well to ensure nothing is limiting, then you slowly reduce the Trace Fe down till you see something negative in the plants, then you bump it back up to the prior higher level.
Sounds simple, but maintaining the other nutrients at high levels and CO2 etc can take some work.
The basis for this is non limiting levels.
So rather than usign a test kit, you take the higher light tanks and see what's dosed there where more trace additions do not help the plant health further.
So say this 5mls per day in a 75 liter tank full of plants with high light/high CO2.
Well, that's a consistent amount you tell other folks to add, right?
You know the plants will not need more than this because it's a very high light fast growing tank.
And this adds the right amount over time, so you can by pass the entire test kit.
Ca and Mg, they vary over huge ranges.
As long as they are not limiting is pretty much the rule.
And as long as they do not build up to toxic levels is the other end of that.
Ca/Mg will very high, as will K+ and Na+ etc, PO4, NO3, etc.
Trace metals like Fe, Cu, Zn, and the nitrogen form NH4 will be very low.