On the version of the Airbus FMGC that we are using at the moment, you can work out an ETP for any two points. The default ETP is from departure to destination, and it draws a nice circle there. But you can replace the departure and destination in the relevant page on the MCDU with any positions (not just waypoints) you choose, and it will redraw your circle.
This is most useful for ETOPS, or ETOPS-like operations (the only time I really ever bother is to work out where the nearest usable airfield is on a long over-water sector - not for single engine work, but for NO engine consideration, I want to be as close to the possibility of rescue as possible). Yes, it's a bit arbitrary - but then, so, really is the idea of having an engine failure between V1 and V2, or the net flight path.