I found that, when considering the gravitational potential energy a particle has (on a universal scale - not mgh, but -Gm1m2/r), it is necessary to consider one direction to be positive and the other to be negative (resolving forces) before converting from force terms to GPE terms.
How does one do something similar for electric fields? The problem is this sort of thing suggests that, the electric field strength on a line between a positive and negative charge never being 0, when you resolve the forces they will not be 0 at any point either, so the electric potential energy will never be 0 and thus the electric potential (which is just EPE per unit charge) will not be 0. This thinking must be wrong because I have various problems where EPE is 0 on just such a line.