How do you determine how much CaCO3 is in the water? Is there a test kit for that? If you have 70 ppm of CaCO3, you have 28 ppm of Ca. Given the uncertainty in the ppm of CaCO3, I would say your 25.5 is right on the button.
Most places where you see the ppm of calcium in water, they express it as if it were in the form of Calcium oxide (CaO). That will be a different number.