Your anemone looks great.
IMHO, When inceases or decreases light, I would change in the intensity of the light, not the duration of it. It he is going to get sun burn from that intensity, then 6 hr vs 12 hr would still causing sun burn or damage. It is possible that the 6 hr exposure cause less damgage than 12 hrs. Now it you give him 1/2 intensity for 12 hrs it would not cause damage, then increase the intensity every few days to let him addapt to it.
I would use eggcrate and put something that would not burn to block some of the light, or raise the light fixture. I would not change the duration of daylight. Just my opinion. I normally don't worry about increase or decrease light level unless I see some problem or unless I know for sure that there is drastic changes.
If your anemone is doing well, which he is , with 6 hrs, then he will do fine with 10 or 12 hrs. I often use 14 hr day light due to the fact that I leave early and get home late. Also I know that I cannot possibly provide as much light as some of these animal used to in the wild so I compensated by increase lenght of daylight. Is there any prove that this works? No, just that my aquarium seem to do fine.