What I learned with my KP.
Buy two CP backs...a .679 and a .685. It'll cost you about $50 for both of them and a tip of whatever length you want.
At that point, you can shoot any paint you ever run across. Anything up and including .684 will shoot just fine out of the smaller back, and anything .685 and larger will shoot fine out of the bigger one.
The testing that Punkworks did a few years ago found that you can underbore up to .007 without any risk of breakage. So you have one back that covers the range from .679 to .686, and one back that covers from .685 to .692. Pretty doubtful that you'll ever end up having to use paint that's not in that range.
I found that, even when my paint was a perfect paint-to-barrel match with my .685 back, it still shoot straighter and more consistently with the .679 back. In my personal experience (not based on anything scientific), underboring gives better results than a perfect match.