I had to go look where I saw that (80/20) thing and it was here
from years ago. And yeah, just because it matches up mostly with my water doesn't mean it is universal.
"Total hardness is the sum of magnesium and calcium hardness. Dividing the calcium level in mg/L by 20 and multiplying by 50, or multiplying the value in mEq/L by 50, will determine the calcium hardness as CaCO3. For magnesium hardness as CaCO3, divide the magnesium level in mg/L by 12.1 and multiply by 50, or multiply the value in mEq by 50. If hardness is not separated into the contributions of calcium and magnesium, you can estimate it by attributing 80 percent to calcium and 20 percent to magnesium (the actual percentages in your water may be different)
. To calculate alkalinity as CaCO3, divide the bicarbonate level in mg/L by 61 and multiply by 50, or again multiply the value in mEq/L by 50."
I have a high school understanding of chemistry that was clouded by a lot of marijuana smoke and an art degree on top of that so, don't listen to me. But that is a very good article FWIW.