Hi John,
I think I know where my old notes are but I'm not getting into the attic at this time of the night!
Yep, from memory I think it's the tangent you describe and for the normal drag/IAS curve I think it was 1.32Vmd that gave you best glide in still air. Offset the y-axis on the x-axis for a head- or tail- wind and keep drawing tangents from the new origins and you get the rule of thumb for gliding more slowly on a tail wind and faster into a head wind.
It all seems a long while ago, however! I've got the mathematical stuff on this somewhere but, er, later...
Regards, Ian
[This message has been edited by Ian Hamilton (edited 10 November 2000).]