Even if you level the reticle, which is what I do, if it's off kilter w.r.t. the turrets then the inputs from the turrets won't be the natural orthogonal vectors implied by the coordinate axes, i.e. the reticle.
Consider an extreme example, a 45 degree can't. You want to move bullet impact one unit vertically. A one unit change on a turret will then produce .707 units of vertical and .707 units of horizontal change.
