I will try this, I did a 3rd test and I got 70. Then I had my wife do it and she got 80.
I would call it 80 then. You can SLAM at 80 but it will be a lot of chlorine to get it up to the level it needs to be at.
So as stated above, your next steps:
- Get your pH up to 7.2 - as stated you can use the pool "pH up" stuff you have. (It's the same as washing soda you can get for a fraction of the price from a grocery store, but since you have it, might as well use it.) Add half, let it mix for 20 minutes with the pump running, test and check, then add the rest if needed.
- Add enough FC to get your pool up to 31ppm of FC. Use liquid chlorine/bleach for all FC additions for the time being. Start brushing/vacuuming the pool as much as you can.
- Multiple times a day (up to once every 30 minutes, but at least 3-4 times a day) check the FC levels again. Get it back up to 31 if it is lower with more liquid chlorine. Keep brushing/vacuuming. No need to test anything but FC during a SLAM.
- When the pool is crystal clear, bring it back to SLAM level just after sunset, and after 30 minutes test it to be 100% sure what that level is. Get up early and test it again before sunrise, and see if it is still the same as it was with the last test. If you lose less than 1ppm, test the CC and see if those are 0.5 or less. If so, let your FC come back down to maintenance levels - about 9-11ppm. You will need to test pH when the FC gets below 10 as above 10 it can interfere with the test. Never let it get below 6ppm.