I would agree that the game doesn't explicitly state that without digging in the code, but if you simply pay attention to how the first sector grows as you build outposts, the "two jumps away from capital" rule is obvious. No guesswork required, just the ability to count to two and trace the hyperlanes out from the system you're thinking of colonizing in non-sectored space.
I've done some experimenting with increasing the sector radius from two jumps to three, and I think I know why Paradox went with two. Going to three jumps opens up the chance that this happens:
Sector 2 is split in half by Sector 1 when the sector jump radius is 3.Code:2 - 2*- 2 | 1*- 1 - 1 - 1 | 2 - 2
You're absolutely right that a carefully observent person such as yourself would pick up on the pattern and adjust accordingly. Not everyone is like you. Many just want to play the game without having to deduce the nature of fundamental mechanics and then adjust their play around those under-the-hood, non-balance affecting aspects. Some just want to play the game and have the information that they need readily available. Some people never go on the forums, read the wiki, or open a single game file and there is nothing wrong with that.
You're right. Two jumps is probably the best setting in order to avoid split sectors. Of course they could set sectors at galaxy generation and avoid that problem (as well as a host of other problems from previous versions) all together.