18.3.1—That, for a point outside a spherical conductor, the charge on the sphere

For a spherical conductor in electrostatic equilibrium, excess charge resides on the surface and the external field acts as if total charge Q were at the centre.

Use centre distance r and remember the field inside the conductor is zero in electrostatic equilibrium.

A charged metal sphere produces an external field decreasing as 1/r², even though the charge is spread over its surface.

The point-charge equivalence does not describe the field inside the conductor or an irregular charged object without symmetry.