Ok, I've had a look. Never used tabs before myself. Firstly as you're in a hard water area, like me, it's unlikely you'll ever need to add calcium as there's so much in the water already, mine comes out the tap at 7.3 ph lowest. So no more cal, I'm thinking that's what caused ya lockout.
When flushing a plant, the 'rule of thumb' is 3 x pot size of water going through to clean the salt build up off the roots. So 11 l pot = 33 ltrs. Seems excessive but if you watch the drainage water from the bottom of the pot it'll go from dark brown to almost clear by the time you've done. Ideal way to flush : Take it in the bath, put it on a wire cooling rack or something similar, just so the pot can drain properly from all holes. Then put a minimum of 25 ltrs through it, don't worry about ph'n all of that. You shouldn't have to ph soil as soil buffers ph anyway. Never tested runoff myself.
Do you have any bottled flower nutes? The nutes suggested to me, which I now use, were Plant magic soil grow hard water, cannot recommend highly enough, if you can get these, get them/order them today.