I'd recommend the 6 o'clock hold for high power. You can get by with a center of mass hold at short range (2-300 yards) but you'll loose the bull if you try it at 600 and 1000.
Another option for 600 & 1000 is the Navy or Frame hold. That is where you push the post up where you have an equal amount of white on the sides and top of the sight post.
This works pretty much the same as using an aperture front sight. The Target frame is a standard 6 X 6 ft at 600 & 1000 yards.
Don't try this if there is snow on the ground, the white target frame blends into the back ground.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071