They say that here in Wisconsin the average shot taken at a whitetail is 35 yards, and from my experience where I hunt, this is correct. I hunt mainly hardwood forests and probably have taken only one whitetail over fifty yard.
From what I understand a .243 is a pretty flat shooter. Find ammunition that will consistently paper well at 35 yards with that firearm and most of the battle has been won. At 100 yards you should be within one inch of zero, or close enough for a clean kill on a broadside or quartering away shot. The vertical kill-zone (lungs / heart) on a broadside whitetail is minimally six inches.
Also, check with the outfitter because now days it is popular for outfitters to have areas set aside for checking zero on your firearm. Shoot Straight, Be Safe, Hunter Joe