There's no set rule, at least in more recent PTs. For example, in PT 81, the second passage was hardest, and the first passage was probably the second hardest.
I just did the "Law of apparent reality" passage Wednesday. It was in my drill set. I found 2 of the questions to be pretty challenging. I have not looked at 81's games, but I'll certainly keep this post in mind.
@amw26 I just took 80 and 81 in the past week. I think they were pretty standard, they were kinda like the 70s tests, but I thought (especially 81) their games were a lot more cookie cutter. PT 80 has one strange game, but it's definitely doable.
Interesting, PT 80's LR felt harder to me than PT 79's by a lot. I would say I felt PT 81 was closer to PT 79 than 80. I think it's all just kind of personal preference, and what clicks for us.