If you remember high school chemistry, you will find this section easy to understand.
Atomic and molecular weights of elements and compounds must be used in calculating nutrient formulation and concentration requirements. Please refer the previous table - Commonly avaliable fertilizer salts, The molecular weight of Ca(NO3)2 is 164.1 (This molecule contains 1 atom of calcium, 2 atoms of nitrogen and 6 atoms of Oxygen). Atomic weights of Calcium - 40, Nitrogen - 14 and Oxygen - 16. So, Mol. wt of Ca(NO3)2 is 40+(2x14)+(6x16)=164.
Now, if a nutrient formulation calls for 200 ppm of calcium (i.e 200 mg per litre) we need 200 mg of calcium in every litre of water. We know that in 164 mg of Ca(NO3)2 we have 40 mg of calcium, so first step in calculation is to find out how much of Ca(NO3)2 is needed to supply 200 mg of calcium.
164 mg of Ca(NO3)2 = 40 mg of Ca
"X" mg of Ca(NO3)2 = 200 mg of Ca
X = 164 x 200/ 40 =>820 mg
820 mg of Ca(NO3)2 will supply 200 mg of Calcium
Since, calcium nitrate contains nitrogen also, 820 mg of calcium nitrate contains (820/164) x 28 => 140 mg of Nitrogen is already supplied while fulfiling the callcium requirement. If the formulation requires 160 mg of Nitrogen, we are short of 20 mg. This can be supplied by KNO3 while fulfilling the potassium reguirement.
This is an iterative process which goes on till the time all the nutrient requirements are met with the available salts.
You could follow these steps - Calcium should be provided by calcium nitrate. Calcium nitrate will also provide nitrate nitrogen. Any additional nitrogen required should be provided by potassium nitrate, which also provides some potassium. All the phosphorus may be obtained from monopotassium phosphate, which also provides some potassium. The remaining potassium requirement can be obtained from potassium sulphate, which also provides some sulphur. Additional sulphur comes from other sulphates such as magnesium sulphate, which is used to supply magnesium needs.
Source: Hydroponic Food Production by Howard M Resh.