Yes you are correct, but if the lights were dimmed about 10-15 mins before actual ldg, I believe this would make a difference.
My experience has shown that it must be an exponential (?) graph as it appears night vision greatly improves in the first half of the 40mins and then slows up.
Got to ride in an A320 last week twice at night for the first time in a number of years and the lights were on full blast. The white roof of the cabin was almost hurting my eyes - a nuclear bomb could have gone off at the threshold on Ldg and I doubt if I would have seen it?