
So by exploring with the realms overlay I could find a border between regions, follow that and mark two points of it on my map. Assuming I know where one Thingwall in either of the regions is, I know that the border is at the midpoint between them and perpendicular to them.
By using a pair of pieces of paper (something with a right angle and something with a straight edge), held up to my monitor it is easy to make a guess where an neighbouring Thingwall is.
That may not be the best description and I can go into more detail if people want/need but it is fairly straightforward to find new Thingwalls using this method.