One approach would be to take a water sample to a pool store and get some test numbers from them without buying anything. Then post your test results and we can tell you what to do.
Read How to Shock (link in my sig). With a CYA of 40, your target shock FC is 16. The idea is to bring your FC up to shock level as often as possible (up to once per hour) until you are done shocking (Water Clear, CC < 0.5, Overnight FC loss <1.0).
What kind of pool do you have? Your CH is a little low if you have a plaster pool.
As svenpup said, you want to add enough chlorine to raise your FC level up to 16. How much that is depends on what your current FC level is and how large your pool is. The Pool Calculator, see the link in my signature, is handy for calculating the amount of chlorine you need to add.
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