Assuming Malaysian Government claims are correct that MH370 dropped to low altitude at IGARI and spent 40 minutes flying back to Pelau Perak in the Straits of Malacca where it climbed again to high altitude north of Sumatra, then the maximum range would be confined by fuel exhaustion to the ring I have centred on Aceh in this image. The small circle near Vietnam is how far the oil rig worker Mike McKay could see an aircraft at 35,000ft.