Here's how I adjust the ocular on a scope.
Some say set the side focus to infinity or crank the magnification all the way up. I've not seen any difference regardless of the side (or objective) focus position, but I set FFP scopes to full magnification so that I can see the greatest detail in the reticle.
I do this during the day, while aiming the scope at the sky so that my eye is not adjusting to bring the FOV into focus.
Unfortunately, if you look through the scope for more than a few seconds, your eye will adjust to bring the retile into focus, so you have to look away for a few seconds so your eye relaxes, then look through the scope; adjust the ocular and try again. When the ocular is properly adjusted, the reticle will be in focus immediately. For most scopes I've owned this technique is all I need to do to set the ocular.
However, I find that some scopes need a fine tuning at the range. Some scopes do not require it, but some do in my experience.
For those scopes (mine were a Steiner Military 5-25 and an S&B 5-25) there's a pretty wide range of ocular adjustment where the reticle appears focused, but there's only one ocular setting where the field of view (target) is focused at the same point in the parallax adjustment as all parallax is eliminated.
What I mean is, if I adjust the side (or objective) focus to until the target is in focus, then fine tune the side focus to eliminate all parallax, it pulls the target slightly out of focus. By holding the rifle steady while moving my head, I can see the reticle shift on the target when there's parallax error present. When the reticle does not move relative to the target regardless of eye position, my parallax adjustment is set. What I did then was tweaking the ocular adjustment until the target was in sharp focus. It only required a small adjustment, and the reticle was still in focus. I never needed to touch the ocular adjustment again. The reticle is perfectly focused and when a target at any distance is brought into focus with the side focus (parallax) adjustment, all parallax error is eliminated.
No doubt some will scoff, but I'm not the only one here at the Hide who has experienced this.