The system needs a smallest digit of X, biggest digit of X and smallest digit of Y, biggest digit of Y.
The format needs to be:
{Lowest point of X, Highest point of X}, {Lowest point of Y, highest point of Y}

So instead of looking for the minimum/maximum X,Y by yourself.
I wrote a simple generator to have less of a headache.
10623 is the minimal of X, 10633 is the maximal of X
9707 is the minimal of Y, 9715 is the maximal of Y.

So you need 4 different coordinates that you can put in the generator.
https://atophite.github.io/RadiatedZonesCoordinatesGen/
You can teleport to 4 different places that makes an imaginary square.
Copy the coordinate of the 4 different places.
You can get the coordinates in PZ Map Project website https://map.projectzomboid.com/
Or you can see the coordinates of your character with a mod like Position XY or view coordinate
And fill in the coordinates in the generator.

Example:
I make a small square of 4 coordinates:
10600x9695
10600x9687
10610x9689
10610x9694

Fill this in the generator.
And you will get this result:
{10600, 10610}, {9687, 9695}
You can now put this example in the coordinates.txt