I think it's clear that at the VFR/IFR and IFR/VFR transition points and levels, flight should be possible under both VFR and IFR. Thus if they are outside controlled airspace, the ceiling at the transition point should be at least 1000 ft above the minimum IFR altitude (usually 1 000 ft above the highest obstacle within 8 km), and if they are inside controlled airspace, the ceiling at the transition point should be at least 2000 ft above the minimum IFR altitude.
Where do you get the 2000 ft figure? The cloud clearance requirements for VMC are 1000 ft vertically in controlled airspace.