HN39,
Extending the curves to the left has solved my problem, thanks. Should have realised that they would rapidly head "south"!
(For those who haven't been following my dialogue with HN39, that enables a tangent to be drawn for any headwind up to the stall TAS.)
Chris
PS
This fascinating discusion has crept well into the theoretical. Am concerned that some readers may not be aware that even flying at the "minimum-drag speed" (roughly speaking, at the angle of attack for the highest lift-to-drag ratio) is not normally recommended on public transport. (I think Concorde, on take-off and approach, was an exception.) And minimum-drag speed is itself way above stall speed. Current jet airliners use a "minimum clean speed" which is just above minimum-drag speed. Flying below minimum-drag speed is a tricky business.