Inverse of the distance squared to be exact.
For a point charge influence upon another point in 3D space the ration would be by the inverted distance squared.
In the case of a single transmission line, say a lossy line receiving the EMP charge from a point charge, the ratio is relative to twice the inverted distance.
Basic field theory.
It gets more significant as the distance approaches the surface. Then half the total charge might be absorbed by the plane beneath it, while the other half might be greatly reflected within the atmosphere.
Next, the field absorption of a space point charge on a distributed mesh network (elec grid).
While many protective relays and fused cutouts could operate in cycles, the concern on the EMP would be to fry the components in less than a cycle.
Original studies by Morse & Feshbach studies in ‘Acoustics’ back in the 50/60s at MIT. Methods of Theoretical Physics