calculate calcium levels from known hardness?

So I have decided my best bet to deal with my local hard water is to dilute with RO. However, a Ward labs report was not done since my area says they change water supplies on an as-needed basis. 2 of the 3 sources are hard, so I am rolling the dice with the numbers I was able to pry from them. I was not able to get a calcium count, but did get a hardness value of 180, and alkalinity (as CaCO3) of 250. Is there a way to figure roughly what kind of calcium I am looking at, so that when I do a 1:2 tap/RO dilution I know if I need to be adding any calcium back?

With a Ward report or a comprehensive report from the utility, you could assess how much magnesium is typically in your water. With that knowledge, you could guestimate what the calcium content is from the hardness value. An aquarium test kit could help quickly assess what the current hardness level is and that can be used to guessimate the calcium level with an allowance for the typical magnesium content.

Well that's the problem. I couldn't get anyone to send me the report, best I could do was get someone on the phone to pull down dusty volumes, and tell me numbers that they could find.

I thought I had read somewhere that you could determine calcium from hardness, multiply hardness by .8 or something. As in, calcium is usually about X percent of the hardness value. Since I know the hardness and alkalinity, I was hoping to be able to pull a calcium level at least close. What are typical magnesium levels?

As I had mentioned, the Ward labs report won't really help, considering how this area pulls water. 2 resevoirs are super hard,and they soften a bit (creating more sodium) and the 3rd is Lake Erie, which has a completely different profile than the others. I was told that they blend...

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.

After mulling it over for a bit, and finding this thread as well, https://www.homebrewtalk.com/f128/richmond-va-water-report-cant-find-ca-mg-water-report-363719/ I think I am going to go the RO route. 1:3 or so, with some small additions of calcium back.

Can't afford an RO system just yet, but I'll start hustling and put it on my wishlist. I would like to at least just try to get close, I'm not sweating exact numbers yet. Thanks for your help guys!