RE: sighting in at close rang
In THEORY it is true...I've heard 23 yards, 25 and 27...
In PRACTICE, you should always try your rifle at different yardages...
I typically sight in about 2-3 inches high at 100...This will put most centerfires with a mv of 2800-3100fps dead on again around 225-250 yards and about 3-6 inches low at 300...
But...You need to shoot your gun with the same ammo you will hunt with to really nail it down...
If you can have 300 yard shots try this...Sight in 3 inches high at 100...Try it at 200, 250 and 300, then you know if you should hold a little low or a little high on a deer...
Even if most shots you take are within 100 yards, I would still sight in (assuming for deer) about 1 1/2 to 2 inches high at 100 yards...This insures you that if you get a shot at 200 you will connect...