So to make fully concentrated kalk solution you add 2tsp/gal. Looks like you are adding 1tsp/5gal? You may not get much return for your efforts with that concentration. I'd do 1tsp/gal and see what your results are.
I use a gal jug, add water and vinegar, then 2tsp kalk. Shake up, wait about an hour and add it to my reservoir.
I also dose with my dosing pump instead of the ATO so a 50% solution makes sense to me adding it that way just to give a little buffer in case of a pump error or something.