FE Hoppy
And how much further could you go at ECON Cost Index 0 for the same fuel load?
If your speed was 0, indeed you're going nowhere, but for the purposes of establishing a search area, I think assuming the speed of the aircraft was 0 is irrelevant.
All anyone can do is:
Look at how much more than Flight Plan fuel was loaded.
Take the FOB (Fuel On Board) figure from the last ACARS position report or estimate it from the PLOG at the waypoint where the ATC handover took place if the ACARS didn't report it, amending for any extra uplift of fuel, pre-departure.
Establish Maximum & Minimum Range markers based on facts including potential crew instigated cruising Mach/IAS variation techniques (ECON CI '0', LRC, Mmo etc.) & 'best guessed' fuel on board for endurance from the last known position.
When new information comes to light such as the aircraft may have been 'Terrain Masking' it would probably add a much smaller 'theoretical' range than originally thought because of the increased fuel burn at low level, ergo reducing range.
Using 'Minimum Airspeed 0' will only keep you at the last KNOWN position. And it aint there.