To figure out the ppm of your fertilizer (or fertilizer mix), you need to be able to measure grams (to a few decimal places unless you are making a lot of solution) and liters. Look at the numbers on the back of a fertilizer bag.
The pitfall to be avoided here is that the fertilizer manufacturers list some of the nutrients as their oxide, like K2O for potassium and P2O5 for phosphate. In the 'guaranteed analysis' there will be a list of compounds, and elements and their % content (supposedly, see conversions below).
For every one gram of said fertilizer in one liter of water, it contributes 10 ppm of the given nutrient per percentage point. A 20-20-20 gives 200 ppm (10 ppm X 20) of each nutrient for each gram in a liter of water.
A=your desired ppm
B=10 ppm X the %(not in decimal form) of nutrient in mix
B=10 ppm X the % of nutrient in mix
C= grams of fert per liter
To convert from compounds to elements:
P = P2O5 X 0.44
K = K2O X 0.83
Mg = MgO x 0.60
You'll need to determine the Ca source to determine actual soluble Ca content:
Ca = CaCO3 X 0.4 for calcite or soluble Ca source
Ca = CaCO3 X 0.23 for dolomite
This is important to know, because the method the fertilizer manufacturers use to label the bags overstate the actual content. This is just the way they label the product, and says nothing about what form the nutrients are really in.
So, to make a 200 ppm N, 100 ppm P, 200 ppm K mix using a 13-0-44 (potassium nitrate), a 12-62-0(monoamonium phosphate), and a 33-0-0 (ammonium nitrate) you would work backwards from your sole P and K sources (it makes it easiest in this case), and make up the N at the end.
You would use 0.55 g of potassium nitrate (200 ppm/(10 ppm X 44 K2O X 0.83)) and 0.37 g of monoammonium phosphate (100 ppm/(10 ppm X 62 P2O5 X 0.44)) in one liter.
This would give you 116 ppm N (10 ppm X 13N X 0.55g + 10 ppm X 12 N X 0.37 g), 101 ppm P (10 ppm X 62 P2O5 X 0.37g X 0.44), and 201 ppm K (10 ppm X 44 K2O X 0.55g X 0.83). 84 ppm are needed to raise the N to the 200 ppm level, so we can use 0.25 g of the ammonium nitrate (84 ppm/(10 ppm X 33 N)) to bring us up to finish.
The actual mix would yield: 199 ppm N, 101 ppm P, 201 ppm K mixture in one liter of water. To get more precision, you need to mix larger batches or get a better scale (you would need to make a 100 liter batch of the above with a scale that is only accurate to the gram).
If you mix your own ferilizer, you can adjust your N source to meet your pH needs (see the FAQ), rather than being dependant on adding acid or base, which is nice.
This works for formulating hydro mixes, as well as for us dirt farmers.