It looks to me like the picture should be rotated so as to Lower the nose just slightly. But anyway you got it accepted now.
The normal checkpoint for this is in the middle of the picture. Towards the side edges of the picture lines are not vertical even if the horizon is exactly level. A common mistake is to adjust with verticals on the edges of the pictures. Then you don´t get it level.
The eye is pretty sensitive to this, so the best thing you can do is to make a judgment on what looks best to the eye, and then let it stay that way.
Pictures with planes climbing, descending or perhaps DC-3:s on ground often lead to misjudgments by inexperienced photographers. Always check the surrounding terrain. Then you get it right most of the time.
Now that you learned, you will probably increase your acceptance rate.