With the OP's scope I'd first remove the rifle from the equation and establish if the reticle is straight or not. You could remove the scope, clamp the scope itself, then do the plumb line method. There should be enough flat on a Nikko top turret to rest a level on it. It should also be obvious by eye if the upper vertical arm of the crosshair does not appear to bisect the top turret.
I imported one of these from the States a few years back
I got it from Brownells
but it is a simple device and a good engineer could knock up a similar gizmo quite easily.
The idea is that a line through the vertical crosshair should bisect the bore. If this is the case (apart from aesthetics) it doesn't actually matter if the top of the action is dead level or indeed if the mounts are slightly offset.
This device also works particularly well with springers and anything else without a flat top. I've never trusted the method of resting the level across the bottom half of the scope ring. As long as the bit of the rifle just in front of the scope bell is round (usually the barrel) it self centres.
It works great and I use it but I have also found that if I do it by eye it almost always tallies with this device. Which is nice.
So these days I set it by eye then check it with this.