There's no one answer. Depends on your attachment method. The one thing to keep in mind is that the rail buttons are there to keep the rocket straight while it is getting up to sufficient velocity for it's shape to stabilize it. So the longer you keep the rocket on the rail, the less likely it will go unstable in the first half-second.
Two ways to achieve this: longer rail, or top rail button farther aft. If you can screw into a CR, great! If you are epoxying to the outside, you have more freedom. Keep them as far aft as possible, but not so close together as to act like a single button.
Let me know if you want clarification. This is the quick and dirty.
A minor crack or split won't hurt the ring in one tiny spot. Even if the screw isn't fully in the ring (or is even immediately adjacent), if you put in a blob of epoxy or CA, it will help support the button.
Now, this sums up all the rail button threads nicely.It doesn't matter all that much so long as they are aligned and on the same section of airframe tube.
Ideally, for minimum weathercocking while elaving the pad, the rear should be at CP 90 and the front should be as close to it as possible. For least stress, there are different locations and for ease there are likely others places as well. Just depends on what you want in your rockets.