In cases where there is a lot of variability in the water chemistry and in particular the hardness and alkalinity then there are really only two approaches: 1) Measure the alkalinity and hardness each time you brew and adjust accordingly or 2) Swamp the variations by dilution with sufficient RO water.
No. 1 is easy enough to do. See http://www.wetnewf.org/pdfs/measuring-alkalinity.html
for the details on alkalinity and there are similar procedures for hardness. The actual measurements can be made even simpler (if a bit less accurate) by the use of 'drop count' kits available from Hach and other manufacturers and at aquarium supply stores. But even so you must do the calculations for the salt additions appropriate to the style given what you measure on a particular day. OTOH sulfate and chloride are more difficult to measure but both are doable and if they vary widely you need to know about that too.
Option 2 requires that you install RO equipment or travel to pick up RO water each time you brew but other than that it is a much simpler option as the required salt additions are the same each time you brew a particular style.
Yes, there is a relationship between calcium ion concentration (mg/L) and calcium hardness (ppm as CaCO3). The former is 20 times the latter divided by 50. If you only know total hardness then you must guestimate the portion that is calcium hardness. 50-60% are probably good guesses but may not reflect the reality of your water.