Here’s the idea,
With your nose on the charging handle, look threw the rear ring, focus on the front post, and let the target slightly bur. Basic stuff, but to zero, you just need to move the rear sight and your eye will move to center out in regards to the rear ring.
As long as you keep a solid check weld to the stock and you groupings are staying tight, it's not a check weld/face-positioning problem.
Now for centering the rear sight for a center zero, you just need to move the front sight post in the opposite direction as the rear to center it out. Either reinstalling the barrel, or just drifting the entire barrel in the upper socket via a leather mallet to the front sight tower, can do this. Best to just go within a few clicks of center since ammo and wind will change the POI/zero from day to day.
Myself, I pick the ammo that I will most likely shoot threw the rifle; find an indoor range (tunnel) and zero out the rear sight via shifting the front tower/barrel. This way, I can use the center point of the rear sight for a repeatable zero, which allows me a point to return to over the course of different distances/different days of firing (environment conditions) and adjust from there.