Yes, a CYA of 50 is fine for a SLAM. You'll increase the FC to 20. You will add chlorine, let it mix well for about 30 minutes then re-test again and increase it back to 20 as needed. After that, you will test the FC only as needed to ensure you are maintaining the FC of 20 consistently. It may be once an hour at first, then it will be longer and longer between tests as your water begins to hold FC better. You'll start to see a pattern. You may only need to test and increase the FC 3-4 times per day. There is no need to test FC in the middle of the night. Get your rest. Once you wake up, you'll increase the FC back to 20 as needed and continue with another SLAM day.
During the SLAM, it is very important to do all the things on the page. Brush daily and inspect ANYTHING that can hold algae. If you have a ladder that can be removed, remove it. If you have a large light with a niche behind it, pull the light out (power OFF). Hollow steps as well. Things like that if they apply. Good luck!