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When you want to increase your free clorine 3ppm, what is a good way to measure it? In other words is 1tbl spoon, 1ppm?
My advice is what In_Canada_eh said with the exception that a CC reading of 3 PPM may be a little chlorine smelly, I would say a 1 PPM CC reading requires a shock. I've only had that chlorine smell once and that was in my pool a few days after opening up and shocking ... I shocked it again to get rid of the smell.
For me, if I add 1 tsp chlorine (to a 400gal tub) after using the tub, I have a FC level of about 3ppm. When I got my tub I read here that approx 1 tsp per user would be a good ballpark of chlorine needed, and that has definitely worked for me.