Re: What ring height?
If you currently have it mounted, measure your current ring height. If your current setup has a zero moa base, then you'll need to figure out approx how much closer the end of your scope will be to your barrel. From center of mounts, measure to front of scope. 1.04" = 1moa at 100yds (300ft, 3600"). Multiply by 20moa and divide by number of inches from center of your mounts to objective end of your scope.
This will tell you approximately how low a 20moa will drop the objective of your scope. For example 1.04*20/3600*12 = .07"
Then get the height of the Nightforce rings off their website. This should get you pretty close.
If you don't currently have the scope mounted on that rifle, then it's a guess. Depending on your barrel contour, how far forward you mount your scope you might get by with low rings, but with the 50mm I'd bet on medium rings (1" or 1.125").
I'd probably call Nightforce and see what they recommended.
If some is good and more is better, then too much is just right.
My mind is a raging torrent, flooded with rivulets of thought, cascading into a waterfall of creative alternatives