I shot my last antelope with the scope set on 10X. He was just over 400 yards and I really wished I had taken time to turn the scope down. The animal filled up way too much of the sight picture for my liking. You need to figure a 'lope viewed at 400 yards with a 10x scope appears to be the same size as one 40 yards away would be if viewed with a 1x optic.
