This floor can be broken into multiple "wings" - inassessible chunks you have to go down and up stairs for. Or find a key to access, whatever.
The quantity of wings (the completely isolated ones that require stairs) you can have is dependent on the number of stairs down of the floor above you and the number of wings it has. This number is StairsDown - (Wings - 1). So if you had four isolated rooms (with one stairway down in each of four rooms) on Floor 1, Floor 2 would have to be one single mass, so you get into every spot of Floor 1.
Now, mulling over what would be the easiest way to implement correct stairway linking - after all you can have two wings from two floors linked only to each other and therefore make dungeons that are completely impossible to complete if you don't do some diligence.
Then another idea jumped into my head: stairway links that go beyond any given two floors. This is when you get into the "fun" realm of having to go down three floors, then going up three floors, to be able to go down four floors and continue on. This thought. Now I have a headache.
I'm not interested in following it of course. jRPG dungeons tend to be simplistic, and the examples of this much assholery (such as in Etrian Odyssey 2) I can think of tend to be short tedius bits of the game.
Really getting an idea of why people don't like making games with randomized elements...