Nice to see they had it right then already
It is tricky though. There is the "minimum amount of light needed for break even" to keep in mind too. Which depends on the species & circumstances under which the tree grew up. And the maximum amount of light untill you max out photosynthetic capacity. Within that range it is beneficial for a plant to get light. How long a dark phase is needed for the plant to "reset the photosynthetic cycle" is again species specific. All this makes it a bit of trial error and guessing. But with generic papers such as that one, some direction is given.
I do not like to run heaters and lights for my plants so an 19 hr day will not happen here. I will add a little light to get to 12hrs, because that is what is in their natural habit the normal cycle.