Well, if CYA is really zero, or close to zero, then adding chlorine during the day is fine at first, but gets inefficient around now.
When you have ammonia in the water, it reacts with the chlorine very very quickly, faster than sunlight can have any effect. During this phase it is nearly impossible to get any FC reading, as it all breaks down too quickly. However, towards the end of the process, which is where you seem to be, there really isn't any ammonia left and instead you are dealing with partially broken down intermediates which react more slowly. At that point sunlight and CYA become significant contributors to FC loss.
It would be interesting to know how things behave after sunset. That would both give us a better read on how far through the process you are and give a suggestion of what your CYA level might be.