Finding a relative #include path is currently a limitation in the implementation of "Add #include", which is a fairly new feature. I will add a feature request to look at improving it in the next release.
@Graeme: This feature can by found by right-clicking on a symbol in the editor and selecting "Imports > Add #include for XXX" or "Import > Go to #include for XXX". For C++, I generally prefer to use "Go to #include, because that way if it adds a #include statement, I can see if it landed where I want it, because C++ #includes can be a bit persnickity unless code is really well formed, which is, unfortunately, one of the rarest things on this planet.