As I said, it's a misnomer.

There is no formula with fixed N here because it operates with a 2^16 lookup table to avoid non-printable characters in both directions.

Edit

So the actual N(X) for a mapping

Y=R(X)

N(X)= N(Y)= Y-X

will vary near approximately 2^15-20+-8 (?).*

And it ignores anything >= 2^16 like emojis, similar to ROT13 ignoring any ASCII outside the alphabet.

*) Actually the description of the author was outdated and doesn't fit his code.

He's excluding 2100 characters.