>I said one could see the FULL surface area only from two directions.
Of course Lee didn't read anything I wrote in reply to David,
which included all the slivers and other lesser areas.<

I surmise that this is very hard to calculate. A 'flag' constitutes a
'sail' (or wing if you like) and creates its own turbulence. It is
therefore unlikely to be 'invisible' from upwind all the time. And it will
also only occasionally be 'fully' visible.

M