Ditto to Wayne, for precision use with a CCO, I dim the dot and use either top or bottom of dot as my "aiming reticle" depending on range.
On magnified optics, depending if there is parallax adjustment on the scope will depend on what you need to do -- some will get you a decent merged image of reticle and target, some will not - and you will need to accept some fuzziness on one or the other (especially with older eyes) - but ideally you are looking at a "flat image" with the reticle crisp on a crisp target.