Its just a matter of using the right target for the same reference of an aiming point.
I was using a leupold vx2, 4-12 when I shot that group. Dont happen all the time, but I did proceed to shoot 3 more groups that were 2", 2.25", and 2.5" at 400 yards. I like to use 10-12" squares, and make a T in the middle of the square for my crosshairs to be placed on. It allows me to center the crosshairs and get the same exact point of aim, shot after shot. This is what works for me anyways.
Usually I have 18x leupolds for longer range work, but I wasn't able to get my ring screws off. I ended up drilling them out and putting my 6-18 leupold on now.
1" at 400 would theoretically be 1/4" at 100, yes. Dont always work that way, but you have the right idea.
I myself am not happy unless I get 2-3" group at 400 yards, even with my factory Rems. They all seem to be more then capable with the right load though, so far...