Couple of suggestions;
First, it is a good idea to check the adjustments at exactly 100 yds. From your zero, dial up 20 moa and shoot again. New POI should be really close to 21" high (20.9"). If this checks out, then the scope is adjusting as advertised. To take it a step further, dial back down to zero and shoot again.......bullets should land right on top of your initial group, dial 20 and shoot again.......bullets should land right on top of group #2 (confirms repeatability).
NightForce should be good, but doesn't hurt to check.
Second, I've found 100 yd zero's to be quite misleading sometimes. What I mean is that are you Really, Truly dead nuts on zero'd at 100
? The scope height is an important thing to know for this, and 3/4 minute groups are tough to tell where your exact
zero is too. Something that may help is shoot at 300 yds before moving out to 450. You can then tell the program where you hit at 300 and it should give you an alternative zero distance. Then run the numbers for 450 and see if they come in closer. You might find a few inches difference after doing it this way.
Third, check your muzzle velocity with a chronograqh. A couple hundred fps faster than you think will mean a sizeable error at 450 yds. A Shooting Chrony is not expensive, and tripods are cheap at Walmart photo section.
Those are the big 3 that are coming to mind at the moment. A combination of just two of those things could easily give you the 8" higher than expected POI.
If it were me, I'd also try to tighten that load up some, but 3/4 minute could just be the shooting rest/position. Did you feel comfortable and really steady??
Oh, one other thing I just thought of........some programs (I've heard) will double compute drops if you input elevation and
pressure. Test yours by leaving the elevation at zero and only changing the pressure VS inputing both elevation and pressure...........see if it makes a difference in generated dopes.