As DE pointed out:
Say your center point is 0 x, 0 z...
Your NE corner is -63 x, -63 z.
Your SW corner is +63 x, +63 z.
(The center block counts as one)
So, basically, stand at the exact center, /getpos, record that (F2 to screenshot if you need to)
Then subtract 63 from both, that is you NE corner. Then sdd 63 to both (the original center coords), that is you SW corner.
So, it would look something like:
Center: X = 0, Z = 0
NE Corner: X = -63, Z = -63
SW Corner: X = 63, Z = 63
Give me all three coordinates, and I will create an output map of your plot... you will need to confirm that you are good with that map before I will export it.

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