What is good enough for a rifle zero? For example using a (x,y) coordinate system, I have never gotten a center group of (0,0) at 100 yards for a ten shot group. So at what point do you say it is zero'd?

For example, I recently zero'd a hog gun using 300Blackout.

A 10 shot group has a center of (-.17,.09) in inches. Converting to polar coordinate, that comes to .2 inches off center at 61 degrees. The Mean Radius is .49 inches. That seems pretty good to me for hogs.

But I am curious when others stop trying to zero and call it done.

With all that said...call it done! the 300 blk isn't a "precision" round...never designed for that. Was designed for CQB with some "brief" distance obtainable with it...

When all of your shots are in the X ring.

What is the diameter of the X ring?

When the group is closer to the point of aim than the scope can adjust for. 

Depends on your chosen sport.  

My point is that X-ring is X-ring, doesn't matter if it's a little off to the right or left, up or down, or whatever.  

Looking at common scope adjustments, 1/4 MOA and .1 mil, the 1/4 MOA is finer than .1 mil, so if you have to be that much closer, a 1/4 MOA scope would do it.  The Nightforce Fixed 42x benchrest scope is 1/8 MOA.

And if your X-ring is a game animals vitals, that's fine too.

There is an answer which would be helpful and measurable. Since OP has his head stuck in a calculator it might be useful. Otherwise N1150x has a very good answer.

Where continued adjustments would only move the Cartesian coordinates for POI from one quadrant to another without decreasing the distance from actual POA to the actual POI. After you adjust your turrets the first time you should be able to drop in another shot and if that shot hits within your normal group size of the previous shot, regardless of direction, you're zero'd. Continuing beyond that is an utter W.O.M.B.A.T. (Waste of money, brains and time.)

Mathematically, I guess when you can execute a shot group and 95% of the time average POI is within 2 standard deviations of your shot group size for the rifle/load combo from POA you can call yourself zero'd. 

You know, I have a page on my spreadsheet that's meant to help people through this sort of unnecessary academic putting from the rough. http://ballisticxlr.com, download (free), use the statistics tab. If you need help, ping me at [email protected].

For me, after a rifle has consistently shot POI = POA at 100yds for me on multiple range trips, I get more confident in it.

My Lilja 318 Grendel build has been like that for me.

If you spend more time with just one rifle for a while, fine-tuning your zero until you get POI in the center of the aiming dot/box/diamond consistently, then it will feel more right to you.

You can also do dot drills to gain more confidence in yourself and the rifle to be predictable.

I'll just add that some scopes make it easier..the vortex razor gen 2's allow you to dial zero without the clicks..Get it dialed close, then loosen the turret screws and then just a simple screw turn under the turrets for the final dial in, then you tighten the set screws and it should be dead nuts centered...

.02...  1 to 3/4 moa for any hunting rifle... sub 1/2 moa  for any target rifle. After that you are just chasing accuracy for the sake of finding out what the gun is realy capable of. Unless you lock it down in a rest to fire it and go through all the other nonsense that goes with chasing .005 poi you will most likely never get there because one of several variables has the ability to change the poi the amount you were chasing.
Plus most people are not physically capable of matching the inherent accuracy of even the most mediocre out of the box hunting rifle. It is mental fuckery akin to chasing consistency in the game of golf.
If you want to find out just how awful a shot you are get a match grade .22lr anything and buy good match ammo. It is an instantaneous learning experience when it comes to the human physical and mental requirements to put lead in the same hole consistently. Optic or not !