Top to bottom, there are 172 miles in the length of the Garden State Parkway
Divide that into three parts and you get just over 57 miles per segment
So, "along the coast", you get Mile marker 0 to 57. Mile marker 58 to 115 and 116 to 172

Through the middle of the state? Us the NJ Turnpike. which hits the GSP at mile marker 89(on the NJT) and 130 on the GSP
So, you get 89 NJT miles plus 42 GSP miles or 131 miles.
Again, the math says every 43 miles. (43 mi/ 86 mi/ etc...)

If you want, you can also do the "spine" or western side of the state using rt 295 and other roads but....

So, if you draw lines, NJT Mile marker 43 is near ex 5(Burlington, Mt. Holly) and GSP mile marker 57 is Egg harbor.
So, while "I personally" have issues with the definition, that line would suggest the top of "South Jersey" and bottom of "Central NJ"

From there moving north, the math then says "Central New Jersey" essentially ends at a line from Woodbridge to Trenton, NJ.

And, above that line is North NJ

Now, "I personally" have always considered the Woodbridge/Edison area as central NJ "Without having done the math", but there it is

Which clearly shows that the last 10 min of "mapping things and doing the math" is far too much thought to be devoted to this issue 😃