WOW! Ray-the-pilot: Making your own formula for reconstituting RO water is a very difficult problem. It is daunting for the PhD chemist.
No, substituting one chemical for another is NOT A DIFFICULT PROBLEM. You do not need a PhD. As a matter of fact it is a fifth grade chemistry class (at least in my home country which is Russia).
If you know the chemical formula of the compound (MgSO4.7H2O for example), you first need to calculate its molecular weight. This means how many grams one mole of the compound weights. You need to sum up molecular weights of individual elements. Molecular weights for each individual element are listed in the Periodic table (see http://www.webelements.com
or fifth grade chemistry book).
For MgSO4.7H2O (which consists of one molecule of MgSO4 and seven molecules of H2O) you should get 12(Mg)+16(S)+4*8(O4-four oxygen atoms) + 7*( 2*1(H2)+8(O))=130 g
For MgSO4 you will get 12(Mg)+16(S)+4*8(O4-four oxygen atoms)=60 g
This means that one ppm of MgSO4 weights less than one ppm of MgSO4*7H2O, so to extract the same concentration of MgSO4 from both compounds you need to take 60/130=0.46 gramm MgSO4
for each 1 gramm of MgSO4*7H2O
You can convert your calcium amounts (CaCL2, CaCL2.2H2O, CaCL2.6H2O) depending on exact compound you have at hand using the same formula.
One usefull tip: Trying to mix different salts can sometimes be a problem. For example, if you mix dry MgSO4 and CaCl2 and then add water, you may end up with insoluble mess ( although I did not try these particular salts, this is true for many other salts). But if you make two separate solutions (MgSO4 and CaCl2) and then mix them together it is perfectly ok (more complex chemistry is involved here though).
I hope this helps.