It doesn't make sense for the number to vary at all. That actually contradicts your own suggestion. If you want it to be predictable, and "regular", you want it to be the same in every window-- more appropriately, you want scrolling to work the same in every application as well. That means it should work according to the system settings. As far as I can tell, it already does this. What is the problem?
As a sidenote, what exactly is unregular about the page up key? Why did you even bring this up? Page up works exactly as it should. It scrolls up exactly one page. This is pretty much the same behaviour you would see in any other text editor with page up functionality. And more importantly, it is completely consistent with mouse scrolling. You're saying it's not, but I really don't see how, since you haven't presented any evidence.
It would help to move this discussion along if you actually defined what you mean by "regular" or "predictable", given that your standards seem to be disconnected from what most users would expect.
