I first heard that described as diametrically opposite.Wouldn't an easier way to say this, would be find the closest part of the cue ball to the object you are aiming, and that would be the center? Because that is what I was taught. The center of the ball, is always least amount of distance from the aim point.
A diameter by definition would need to be measured through the ball center.
A method I learned concerning aligning the cue, the vision center and the ball center involves the reflective surface of the cue ball.
If all are aligned, the cue shaft is mirrored in the cue ball, and the reflection is in line with the shaft.