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900g per 2490 litres of water = 0.361 g/l (3dp)
Sounds about right for very hard water. Most of London will be similar. (I know you're not in London.)
https://wholesale.thameswater.co.uk/check-the-water-quality-in-your-area
Putting in my SW15 postcode I see I'm served by Putney Heath (no surprise as the reservoir is about 1km away from me and up hill from here) I get a result of CaCO3 of 259 ppm (which is also the same as mg/l).
Buy shares in whoever makes Viakal.
Your water softener removed 12Kg of rock in 4 months?
3kg of rock per month?
1kg of rock every 10 days?
WTF is wrong with your water supply... and a sieve would probably be cheaper.