Use calipers and measure the outside diameter of your scope's objective assembly. Divide by two. This is the dimension above which your rings and base need to raise the scope to ensure it will clear a barrel the same diameter as the rifle action.
Most barrels have a taper, and are not as large in diameter as the action to which they're screwed, so you'll have a safety margin,clearance) in this respect.
Better the scope is too tall than too low. Your scope must NOT touch the barrel, and you might wish to use lens covers which have a certain thickness. In my opinion, about 1/8" is nearly perfect clearance between the objective assembly and the barrel. More than 1/4", and I'm not happy. Good cheekweld is key. If your rifle stock has an adjustable cheekpiece, then a scope sitting a littler taller than might be ideal is not nearly as much of a problem as a fixed stock.
If you use a tapered base, keep in mind that a 20 MOA taper is only 1/3 of a degree. Even so, this will tilt the front of the scope a bit down, which is probably not as much of a slope as the taper of the barrel any way. Not to worry.
Figure out which base or rings you want to use, then choose the remaining component,rings or base) accordingly. Terms like "low", "medium", etc. are worthless unless you know the corresponding heights, which are different among every mfg.
In most cases, shooters will mount their scopes too high, so once you decide on a base or rings, feel free to ping us back and we can probably help you choose the remaining component.
If it were me, I'd get the Nightforce with a 50mm objective instead of the 56mm. Those big scopes weigh a LOT,2 lbs!)
Good luck, whatever you pick.
