how did you come up with the +/- 20 number? EDIT: nm, I see the size of the sprite is 20x20
No, it has nothing to do with the sprite size, just some non-zero number.
Yeah, rotation certainly makes it harder to pick the center point. I would probably do this with LOS behavior on the posHighlight sprite. Set LOS cone of view to something like 60-120 degrees. You'll have to experiment.
When W is pressed, set posHighlight angle to 270 (up), pick all dots in line of sight, and then pick nearest. When A pressed, set angle to 180 (left), and do the same. This should allow to pick the center dot.