You make 2 observations, let's take them one at at time.
1- When you have the side focus set to infinity, everything appears to be in focus when looking at targets 300+yards downrange. This is due to an effect called depth of field, or DOF for short. DOF is the range in front of an behind the setting of the focus in which the objects appear to be clear enough to be resolved by your eye, or more specifically, the objects you are looking at are bigger than the circle of confusion (CoC) for that setting. This DOF is a function of magnification distance and size of the aperture. The further away the focus is, the deeper the depth of field. The higher the magnification the shallower the depth of field, and lastly the smaller the aperture the deeper the DOF. Now that last on is not something you can easily control in a riflescope, that's more for camera lenses, but you can use a reducer to reduce the aperture (the objective lens) and get a deeper DOF.
When you get past a few hundred yards with a fairly low magnification optics like the M7Xi, setting the side focus to infinity will produce a deep enough DOF that you think everything past 300 yards is in good enough focus.
2- At infinity, you cannot detect any parallax error for objects at 300 yards or more. The formula to calculate parallax error is: ((0.5 x D x (abs(R-P))/P ) x 0.0393701). That will give you inches. If you want it in mil, just divide the result by 3.6 and then again by (R/100).
D is the diameter of the objective lens in millimeters, in your case 56.
R is the range to target in yards , in your case 300.
P is the parallax setting, since you put it at infinity, let's use 1000yards.
So at 300 yards, the error is .771 inch. That is the maximum error you could see in your scope, if you could not line up behind it at all. If you have any marksmanship skills, that error will be less. At 400 yards, the error is .661 inch; at 500, it's .551 inch; at 600yards, it's .440 inch and so on. This is very small and pretty much undetectable in your optics.
I sure hope I didn't drop a decimal somewhere, but that's the story.
Just for grins, I did a quick computation for parallax set at 100 yards. The error at 200 yards is 1.1 inch; at 300 yard, 2.2inch; 500 yards, 4.4 inch and so on. At 1000 yards, it would be almost 10 inches.
