I think you misunderstood what I meant.
The IAS VNE at cruising altitude of 20000ft is 272kts
(assuming as I said before that IAS VNE is at cruising
altitude) . This equates to a TAS of 364kts. Which
means that VNE is 364kts TAS.
So in this aircraft at sea-level you could technically
go to 364kts IAS and still be below VNE.

again I'll re-iterate that VNE is influenced by TAS
(and Mach Number) NOT by IAS, which is merely an approximation
of TAS valid at sea level.

Your calculations were fine, it was just the assumption
that VNE would be based on sea level conditions rather
than cruise conditions that i think was wrong. I could
be wrong too, but it just seems logical that for an
aircraft that spends most of its time at 20000ft the
VNE should be based on IAS at this altitude to make
things simpler for the pilot.

Regards,
Jon.


p.s. Dont get any ideas about going above IAS VNE at
sea level in your glider - its VNE is normally based
on IAS at 5000ft leaving very little margin.