Generally, you should connect the two contact points. In some cases, you can aim directly (CB center) to the contact point (assuming farthest spot from pocket, opposite side on target ball), but not always. The contact-point-to-contact-point is the geometrically desired, "center-pocket" aim, but there is also 'throw' to contend with. However, if you have a fairly consistent, baseline result in your shots, then you can learn to compensate for speed/spin differences.
The point-to-point aiming is to offset for the curvature of the balls, which is not considered when aiming center-CB to OB-contact point. The offset is compensated for in other aiming systems, such as using the "side of the cue (ferrule)" type of alignment, which moves the centerline of the cuestick over for the necessary offset. For Ghostball, the offset is achieved by aiming at where the centerline of the ghostball touches the cloth. For fractional aiming, there isn't really a calculation for offset, as the specific cut-angles are "burnt in" by practice, and more/less cut angle is determined by experience.
I learned to offset by aiming (CB) centerball (to *assumed aimpoint to make the ball*), seeing that I missed, and then began to move my aimpoint to compensate *to the pocket*. I have other ways to determine cut angles now, but that was how I began to focus on *making the cue ball hit the "outside" of the target/object ball*.
