What is the best way to calculate the mass percentage of Calcium Carbonate from water? Where I live it's believed that local tap water is really bad for drinking because of the amount of limestone. As a science experiment for school I wanted to approximately calculate the percentage of calcium carbonate in tap water so I can make the conclusion: Is that amount of drinking $\ce{CaCO3}$ actually the problem or something else?

This question might be similar to DK4's question but I'm trying just to calculate not extract, only by using items you can find in a typical house (or in a local shopping center).

If calcium carbonate isn't the issue what is?

Edit: As pointed out in comments, percentage of $\ce{Mg}$ is also applied.
Water contains negligible amount of $\ce{CaCO3}$ or $\ce{MgCO3}$, as these are almost insoluble in water.
What you are interested in are bicarbonates $\ce{Ca(HCO3)2 + Mg(HCO3)2}$. Their content is known as the temporary or carbonate water hardness that can be expressed as $\ce{CaCO3}$ equivalent.
The complement to the carbonate hardness is the permanent hardness, formed mostly by $\ce{CaSO4, MgSO4, CaCl2, MgCl2}$. The sum of the temporary and permanent hardness is called the total hardness.
Bicarbonates ($\ce{HCO3-}$) are not threat to health themselves, as they convert to $\ce{CO2}$ in the stomach and are present in blood as the component of the blood $\ce{pH}$ buffer.
What can be a subject of health concern is the total content of $\ce{Ca^2+}$ (and in lesser extent $\ce{Mg^2+}$). It is again expressible as $\ce{CaCO3}$ equivalent.
Calcium/magnesium content is not measurable by resources usually found in households or local shop center.
The best way is to ask the water provider for the data.
The 2nd best way, close to home-like determination, is to obtain the set for aquarists from a pet shop for determination of carbonate and total water hardness. I am a former aquarist, and this was available even in 1980s in the former communist Czechoslovakia.
Determination is done by counting  the drops of the colored solutions, mixed with the tested volume of water, until the color changes.
Carbonate (in fact bicarbonate) hardness determination is based of reaction of an acid with bicarbonates: $$\ce{HCO3- + H+ -&gt; H2O + CO2 ^}$$
Drop counting ends when yellow color (methyl orange dye indicator) changes to red. The dye turns red by the first excess of the added acid, as has red color in acidic solutions.
Total hardness is based on reaction of $\ce{Ca^2+ + Mg^2+}$ with ethylendiamintetraacetic acid (EDTA) in alkaline solution, enriched by the addition of the dye indicator Eriochrome black T, 
$$\ce{Ca^2+ + H2EDTA^2- -&gt; 2 H+ + CaEDTA^2-}$$ The EDTA first reacts with free $\ce{Ca^2+ + Mg^2+}$. The drop count ends when red turns to blue, as EDTA captures $\ce{Ca^2+ + Mg^2+}$  bonded to the Eriochrome black T.
As result, you obtain the carbonate (temporary) and total (temporary + permanent) hardness in 2 possible units: $\pu{mmol/l}$ or $\pu{dGH}$ (the degree of General Hardness).
$\pu{1 mmol/l = 5.6 dGH}$.
$\pu{1 mmol/l}=\pu{100 mg/l}= 0.01\%$, as molar mass of $\ce{CaCO3}$ is cca $\pu{100 g/mol}$.
The typical total hardness of drinking water is $\pu{1\!-\! 3 mmol/l}$,  i.e. the equivalent of $\pu{100\!-\! 300 mg/l}$, resp. $0.01\!-\! 0.03\%$ of $\ce{CaCO3}$.