I will do my best but even practice at a range is not like in the field, after a lifetime of not shooting past 100 it will be tough to ever feel comfortable out that far - but I will try. Also after a lifetime of bowhunting, and occasionaly shotgun & muzzleloader out to 100 yards, I am used to letting game go by at any range over 150, so it will be ok.
I never shoot unless I am 100% sure I am putting one in the boiler room, and I just would not feel that way at 400 yards (but might). I use shooting sticks and again not having any ranges around past 200 make it tough. I can find longer ranges but to practice there will take a major effort, like an all day affair, so I may not be able to do it enough to really get used to it.