[@Archangel]

The 2:1 applies if you already have a 16 or higher. If you do, any points that would raise another stat above 15 become 2:1, but otherwise, you've got it.
To put it another way, you can only raise one stat past 15 on a 1:1 basis (up to 18 for that stat, still on a 1:1). Any others are 2:1.

As an example: these are my PC's stats before racial bonuses are added,
8 STR, 17 DEX, 14 CON, 10 INT, 15 WIS, 14 CHA.
As you can see, I didn't do any 2:1 stats, because I find that to be a massive waste, but for min-maxing purposes, I could have lowered CHA to 10 and made WIS another 17. However, that didn't fit with the character's intended role and overall personality and capabilities.