Hi and welcome!
What Richard (Chem_Geek) is trying to tell you is that your emphasis is potentially dangerous. Clear water is great, but there can still be dangerous contaminants in it.
Therefore the relationship between the Free Chlorine level and the CYA level is crucial to safety. When your CYA is 70, you need to be maintaining your FC level between 5 and 10ppm, not 1.5-3ppm. It's just not safe. Our "Best Guess Table" is really the way to go to stay safe on the relationship between FC and CYA. IF you monitor your FC levels and adjust them, you can safely maintain water with a CYA as high as 100ppm.

And, personally, I think that a higher CYA when you close for the winter isn't a bad thing as it will probably drop, but in the spring you may have a higher level than zero.

Tri-chlor tabs can be great when your CYA is low and your pH is high. They are acidic and add stabilizer. But if your pH is on the low side, and your CYA is ideal or even high, they are a bad idea.

As for your calcium levels: If you have a vinyl-lined pool, you don't really need to worry about calcium until it passes 400ppm or even 500ppm, and only then if your Total Alkalinity is in the 180-200ppm range. Beyond that you make get scaling and cloudy water. The first answer is to cloudy water (from calcium, not algae), of course, get your T/A down to the 100 range and see if it clears up. There's no reason to drain water when calcium hardness is at 300ppm. If you have a concrete, tile, plaster, etc type pool, calcium hardness should be maintained between 200 and 400 ppm, and Total Alkalinity kept under 125ppm. But, again, a calcium hardness of 300ppm is fine.

Bleach/liquid chlorine. It WILL mess up clothes if you aren't careful. But it doesn't have to. 15% concentration is higher than we get in the USA--usually the pool stuff is rated at 12.5%, but if it's fresh and new, I've tested it at 14% frequently. At higher concentrations, you need to keep it cool and dark as it WILL degrade more rapidly than lower concentrations. Ordinary laundry bleach should be available in Spain and is easy to handle. It ONLY adds FC, and doesn't affect CYA or pH. That is why we recommend it.

A final note: If you use a salt water chlorine generator (I do) your relationship between FC and CYA needs to be at least 5% (much lower than the Best Guess table) because the constant and steady infusion of chlorine into the water. So if your CYA is 70ppm, you need to keep at least 3.5ppm of FC--lower than the recommended 5-10ppm. Plus, most systems recommend a CYA of 60-80ppm.