rear sight out of plumb

[Kurt]

This is were a wind gauge sight comes in if you shoot matches with the rifle when shooting sighters when allowed I take note how the mirage is moving. I set the tang on zero and adjust the front sight. That way if the wind starts to fish tail I always know my zero wind setting.
In your case what I would do if your rifle has a spirit level on the front I would use it to plumb the tang because that is what you will use when you shoot. If you want to get the tang zero drive your front sight in the direction you want to hit. With you only being off 1"@100 yards you wont move the front much more then 1 or 2 hair width or take the base off and use some 220 emery and take off a swipe or two with the cloth laying on plate glass and check where the impact is following a shot. I do this at the range if the rifle does not have a wind gauge front sight. That base will always stay with the rifle.
With these rifles just a whisper of a slight breeze that will just raise a feather hanging on thread will change the impact at 200 yds. It might be on the back of your neck where you are but that does not mean it is the same down range.

Kurt