It's centered until you use the auto collimator, you say. From the image, it looks as though your diagonal might have to be rotated a bit or moved "up" a bit. But, if you rotate it and use the auto collimator, then you'll likely rotate it back to that location. Maybe a bit of movement "up" might actually "center" it a little better.

I don't use an auto collimator, so hopefully someone who has experience with them will reply. But, from an "eyeball" point of view..."up" might seem the way to go.