Greatbear:
I'm sure the Brazilian/French team has spent hundreds, more likely thousands, of navigator man-hours best-guessing the "Ultimo Reporte" location and the debris drift.
they did not "best guess" the "Ultimo reporte"
This is the last VHF transmission of reporting point by the crew before leaving the Brazilian controllers. It is established that this report was made at 01:33 estimating TASIL at 02:23.
Though I agree, the search was initially launched by applying current/wind drift patterns.
Source: early BEA briefing on french TV