Re: focal planes
To your original question:
Thee are 2 different images in your scope. The image of the target, which is focused by the side focus (move the image focal plane forward and backwards, with the idea that it is focused sharply on your retina). The second is the image of the reticle. If the reticle image plane and the target image plane do not coincide, then the point of impact will shift if you move your eye off the exact optical centerline of the scope.
To avoid this state of affairs, the recommendation is to point the scope at a featureless surface (plain untextured wall or open sky with no clouds). Then position yourself behind the scope, close the eye, and open it briefly to see if the reticle is in sharp focus. Adjust the eyepiece focus in or out to get the reticle to look as sharp as possible when viewed briefly (if you stare at it, you eye will try to correct for out of focus and fool the brain). When you get it as good as possible, then point at a high contract target and adjust the side focus until the target is sharp. Probably want to do the same routine (look briefly through the scope, then close the eye, repeat) since if you stare at the target the brain will try to correct.
When the target appears to be in focus (without having adjusted the eyepiece, only the side focus), now align the rifle carefully with a spot on the target and fix the rifle position (gun vice or whatever means you have). Now without touching the rifle or changing the point of aim, move your eye away from the center of the scope and see if the position of the cross hairs moves off the point of aim. Move the eye left, right up / down to see that it stays aligned with the point of aim. If it stays aligned, then all is well for that target distance. Now the procedure has to be repeated at other distances. Obviously at long range it make a bigger difference, but the good thing is that as one approaches infinity the degree of focal shift becomes smaller and smaller on the side focus, whereas a relatively large correction is needed for intermediate distances (100-500yds).
One of the things to check is whether the scale on the side focus matches in any way the actual distance to the target. Len says that it generally does not, even on high $ scopes. That means one needs a kind of "cheat sheet" for the scale on the side focus wheel, because in the field one seldom has a high contrast target, so after rangeing one needs to simply rotate the side focus to match the range as quick as possible.