In a long English secret, the most common letter is x…

The rule: the commonest letter stands for e, so the shift is the distance from e to it, wrapped with % 26.

• x is place $23$ (counting a as $0$) and e is place $4$. The shift is $23 - 4 = 19$.
• b is place $1$. Then $1 - 4 = -3$, and $-3 \bmod 26 = 23$. The shift is $23$.

Python's % gives a positive answer, so (1 - 4) % 26 is $23$.