Thanks for all of your info, if you don't mind me asking, how did you work this out? I am struggling with this part haha.
Ok, can do, but the chemistry can be confusing if you're not used to it.
Magnesium has an atomic mass of about 24.3 and a 2+ charge, so an equivalent mass of 12.15 (24.3/2).
Calcium has an atomic mass of about 40 and a 2+ charge, so an equivalent mass of 20.
Calcium carbonate has an atomic mass of about 100 and the carbonate component has a 2- charge, so an equivalent mass of 50.
To get Mg hardness as CaCO3, you first work out it's concentration in mEq/L by dividing it's concentration in mg/L (or ppm; they're the same) by it's equivalent mass. So Mg (mEq/L) = 5/12.1. This then gets multiplied by the equivalent mass of Calcium carbonate. So, Mg (hardness as CaCO3 = 5/12.1x50 = 20.66. Close enough to 20 for our purposes.
Total hardness as CaCO3 is a combination of Calcium and Magnesium hardness as CaCO3. Total hardness is listed as 242ppm, so Calcium hardness must be about 242-20 = 222ppm, which is close enough to 220ppm to make things easier (again, I'm rounding as all numbers are estimates anyway). To get Calcium (mg/L) you do the reverse of what we did with Magnesium: divide by the equivalent mass of Calcium carbonate then multiply by the equivalent mass of Calcium. So, Ca (mg/L) = 220/50 x 20 = 88.
So your water actually has about 88ppm (or mg/L) Calcium! My mistake with the Calcium calculation in the first post was twofold: firstly, multiplying by 12.1 (for Magnesium) instead of 20 (for Calcium); secondly, trying to do this at 1.30 in the morning after several beers!
With 88ppm Calcium, you don't need to add much more for most ales, and might even consider cutting your water with RO for some pale lagers.