I bought one of the Brompton cranksets from CM Wasson,
put it on with a 15 t cog on the old AW3. the 15 came from the Mk3 sram 3x2 hub spares

end up with a 77/30, 58/23, 43/17 combination
using Sheldon's calculator.
on the foldabikes site there is a little different math used on his gear calculation.

It works OK for Me.. the 43 to 30 double shift doesn't take long ..