Creating a chemical solution with water and chlorine I bought a swimming pool for my kids and also a measurement instrument for measuring the amount of chlorid in the pool in mg/l. The problem is now that you have to calibrate the instrument before using it. This means you have to know the current amount of chlorine in the pool.

Since the test strips are quite inaccurate and hard to read an exact value. My idea was to just create a perfect solution in a one liter water container.

Now I am not really sure how much of the given pool chlorine I have to put into the 1 liter water to get a 1.5 mg/l solution of chlorine. With that I would be able to calibrate the instrument perfectly.

What I am using is chlorine granules.
The instruction leaflet says it contains "CAS 51580-86-0 Troclosenatrium, Dihydrat (100g/100g)"
Registration: BAuA N-23822.

Would I just have to add 1,5 mg to 1 liter of water to get 1,5mg/l?
Is it maybe that easy?
Is your measuring instrument made for measuring chloride or chlorine ? It is not the same. In your first sentence, you speak of chloride. Later on you speak of chlorine. Anyway, the best thing to do is to buy a bleach solution. The amount of chlorine is printed on the label. It is often $14$% chlorine $\ce{Cl_2}$. By diluting a sample (maybe $1$ mL) of such a solution in 1 liter water, you obtain a solution $0.14$ gram chlorine $\ce{Cl_2}$ per liter, and you can dilute it still more.