Its an optical illusion at times though.
Same as hanging a picture.
You can measure it out and it's mm perfect on a tape but crooked to the eye,
It can be done via the eye and seems straight but via tape ...it's out.
In the end, put the gun down, place the level at the breeech, make the gun level and set the scope a per guide.
It may well SEEM off but i bet it isn't.
Hard to explain i know or understand, i had the same thing with my Venom HW80k years ago..Drove me mad a full evening.