Welcome to the forum :lol:
Keeping your posts in one thread will get you better answers and help us better understand your questions.
The purpose of the OCLT is to determine if you have properly shocked the pool. So, when you pool water is crystal clear, perform the FAS/DPD test that evening. Note your FC and CC results. CC's need to be .5 or less. If they are not, you need to maintain FC at chock value and you can stop the OCLT.
Let's say, however, your water is clear and your CC's are .5ppm or less....you have met 2 odf the three criteria.
Now you want to see how much chlorine you will lose during the hours of darkness. By eliminating loss of FC from the sun, any remaining loss can be attributed to algae still in your pool. Loss of 1ppm or greater (during the hours of darkness) of FC is too much so you fail the OCLT and should continue to maintain shock level.
Or, more simply, the purpose of the OCLT is to measure the FC loss during dark hours.