I'm in the same boat as you, using 220/240V with no way to measure. Indeed, that number is just on the Cube side, but I haven't seen any differences between firmware versions. My worst Neptune runs at 1,400W with undervolting, so 1,700w sounds about right for 6 Cubes.

Someone on the now nuked KnC forum (either shyliar, or prototype) had written up how to estimate total usage at the wall, but I don't remember it. I ended up at a 1,900W estimate at the wall for the Neptune showing 1,400W on the web interface. I just use the % difference between those numbers to calculate my other Neptune's usage. It must be fairly accurate, because I have 15 Cubes running on a 30A 240V circuit and the breaker stays fairly warm. That should be running right around the recommended continuous load max of 80% on the breaker.

If you don't mind burning money (like some "Einsteins" around here, lol), you could get one of these to get an actual reading at the breaker box: http://www.amazon.com/Fluke-323-True-RMS-Clamp-Meter/dp/B00AQKIEXY/ Of course, there are cheaper brands, but they all look like cheap junk after reading the reviews.

Did you accidentally go the wrong direction on the voltages? A higher negative number equals a lower voltage. So -0.0439V is lower than -0.0366V.