So the thing to understand about ground water/drinking water testing is that municipal facilities are required to test for A LOT of different chemical species and while there are quantitative chemistry tests that can get them the answers, training people to do those tests would be a nightmare. So they almost always rely on mass spectroscopy as a testing method because the output of those machines is the exact concentrations of almost any atomic species including some organic molecules. Those tests will generate concentrations for the exact atom in question. So if you look at the data where the mass of magnesium is located (24.305 amu), you will get the concentration of magnesium atoms in mg/L or ppm. Same is true for calcium (40.08 amu) or any other element.
When testing for calcium using the wet chemical tests like the titration test in the Taylor kit, all of the reagents are based upon a standard chemical. You simply choose the standard basis that makes the most sense for the test. For calcium hardness, all of the reagents are standardized against a solution with a known concentration of calcium carbonate (CaCO3). So, when you are titrating with the R-0012 drops, every drop of the reagent will chelate exactly 25ppm worth of calcium carbonate in a 25mL water sample. So no matter what is in the water sample (eg, calcium only, or calcium and magnesium), the answer you get from doing the titration is always reported in units of CaCO3 or, as chemists like to say, equivalents. Now in the CH test, the protocol is designed so that only calcium is affecting the color change of the reagent and so then you are assured that you are measuring calcium hardness alone.
Now we go to the mind-numbing details of chemistry. One of the most consequential discoveries of chemistry is this - no matter what the chemical species is, a "mole" (written, mol) of any substance contains the same number of individual atoms or molecules as any other substance, that is, Avogadro's Number (NA = 6.023 x 1023 atoms/mol). It's related to the physical theory of the "conservation of mass" - in any chemical reaction, the masses of the substances involved never change, they simply get rearranged. Because of of this fact, one can relate quantities of substance in any chemical reaction to one another very easily. If you have 1 mol of calcium, it weighs 40.1 grams. If you have one mol of magnesium, it weighs 24.3 grams. These physical properties NEVER change.
So why does that matter?
Because now you can take the information you have about the measured amount of calcium in your water as Ca atoms and convert that easily to what that would be if it were measured as calcium carbonate. In other words -
65.6ppm [Ca] = 65.6 mg/L [Ca] * (100.1 gm/mol [CaCO3]) / (40.1 gm/mol [Ca]) = 65.6 *(100.1/40.1) = 65.6 * 2.496 = 163.7 mg/L [CaCO3] = ~ 164ppm [CaCO3]
The same can be done when converting magnesium ions into the effective magnesium hardness in equivalents of calcium carbonate -
24.3 x (100.1/24.3) = ~100 mg/L [CaCO3]
TH = CH +MH = 264ppm
A simple analogy would be to consider what is being done here like driving over a bridge from the US to Canada - your velocity doesn't change but you can either read your speedometer in units of miles per hour or kilometers per hour.
As far as the autofill goes, your pool water will be more concentrated with calcium than your tap water because the water evaporates and leaves the calcium behind. So unless you get very significant rainfall that overflows the pool, the CH will always increase.
