Hey kinetic! Make up a batch of 1000 total grams of dry ferts. Each 10 grams would equal 1%. So to get .15% iron you would add 1.5 grams
That doesn't actually work because you are not adding nutrients in their elemental or pure form. Pretty much any nutrient you add will be bound up in salt form.
Use the periodic table and the empirical formula of the compound to determine the percentage of mass of the element you want in the salt you are using. Divide the mass of the element that you need in the mixture by the percentage composition of the element and you now know how much total mass you need to add of that particular salt.
For example, if you need 5 grams of calcium and the percentage of your salt that is calcium is 20%, then you would add 25 grams of that compound.
The easiest way to do this is to make the formula so that it easily divides into an even number of doses that you plan to use in your tank.
Lets say you are going to dissolve this into a 500mL flask of water, and you want to get 10 doses per mixture. You will need to add enough of the element to produce 10X the concentration you want to dose in your tank and then dose 50mL each time. If you want 10ppm or mg/L, in a 100 liter tank, then you will add 1 gram at each dose. You now know that you need 10 grams of that element to add to your solution. So if you were using the calcium example above, you need to add 50 grams. Do this relatively simple calculation for each nutrient.
Once you know what the total weight of a dose will be, you can multiply the formula by any amount to keep a dry mix handy.
Doing it this way prevents you from having to use a more complicated equation to determine the percentage without knowing what the mass of the final solution will be.
It's easier to figure out if you make the liquid volume of the container something evenly divisible by 10 also.
It's also better to put all of the dry salts into the flask, then fill with water to 500mL. Otherwise if you start with 500mL of water the total volume will increase when you add your dry mix.
Wow. This is so much easier to just do than it is to explain.
This is math that we learned in the fourth grade, it's just difficult for many people to apply to real world situations because they aren't accustomed to thinking in this way.
Pretty much all of our dry ferts will be hygroscopic also, meaning that they will contain a decent amount of water, forming pentahydrates and the like. This will have an affect on the weight of the dry fertilizer, lowering the percentage of the nutrient we want. When I do rough calculations, I just ignore this, as ultra precise measurement is totally unecessary for our purposes.
By the way, using this method, you can easily make your own clone of any commercial fertilizer you can buy, whether agricultural, aquatic, or hydroponic, using the ingredient list and the guaranteed analysis required to be printed on the bottle by the Department of Agriculture.