I was a bit puzzled as to where the 20:40-40:20 came from, In the old days of film, a feature consisted of eight 15-minute reels, corresponding to 15x2 :(15x2)-(15x2): 15x2. But, yes, viewers don't need a lot of build-up, now, and unless there's a really peculiar ordinary world to establish, 25:30-30:25 will do the trick, and even less in horror, etc.

Jeff, there are so many formulas and paradigms floating around, hard to say what the source of the 20 - 40 is. And yes, you are right, Jeff, that the emergence of two-reelers, three-reelers, and so on in the first decade of the 1900s led to the evolution of 'sequence' writing, each reel having its own beginning-middle-end, the latter a cliffhanger to sustain the audience's interest while projectionist changed reels. Eight sequences is a common approach to screenplay structure taught at film schools like NYU and USC. And for a path of least resistance re Hollywood, that's a safe approach. However as I said in the OP, we always need to respect the specific needs of each story as they are organic in nature and require the freedom to 'breathe' their way into existence and not be forced into a formula. That's one big reason for my mantra: Begin with characters. End with characters. Find the story in between. Because if you immerse yourself in the lives of your characters and follow them into and through the story-crafting process, no matter what structure you end up with, even a traditional three act / eight sequence approach, the story will have a vitality of its own because the characters have come to life.