Hold on, are you sure this is correct? I'm not a doubles player, so I can't test this or confirm this from personal experience or anything
so please do say so if you're sure and I'm mistaken, but on looking it up, I've found that it applies "
if multiple Pokémon are present on the user's side of the field" and "
[Reflect and Light Screen work] in doubles too, but in the case of having two Pokémon out it will only take 33% of the damage off." It
looks like it really does depend on the number of Pokémon on the field and isn't fixed to the 2/3 the whole time.
That said, its Smogon entry does just say "in a double battle," and Serebii doesn't mention it at all - and to be honest, my source being Bulbapedia and one podcast I've never followed before... doesn't lend me a ton of confidence in it. If you have any reason to believe otherwise, you're probably right! I just want to clarify whether this is based on testing or experience or if you're just making a guess based on Smogon's wording or extrapolating from other effects' precedents without actually knowing this specific case.