 Smooth rational-curve self-intersection on a projective hypersurface

For a degree-$d$ smooth surface $X\subset {\mathbb{P}}^3$ and a smooth rational curve $C$ of degree $a$, the canonical bundle is $K_X={\mathcal{O}}_X(d-4)$. The adjunction formula gives $(K_X+C)\cdot C=-2$, hence $C^2=-2-(d-4)a$. In particular a line has self-intersection $2-d$.