this is a good observation - I don't think monk wind stance gets any bonus to offhand attack % rate with the new table: the 80% that monks get is from taking all the twf feats (base 20% + twf 20% + itwf 20% + gtwf 20% = 80%)
I would guess that it's 2.5% insight bonus to double attack rate per wind stance but Eladrin would have to confirm and update the OP.
A potential option to make up for the fact that monks are unable to take stwf (because of BAB being too low) would be to give each wind stance a 5% bonus to offhand rates as well as a 2.5% bonus to double strike meaning a monk with gtwf in wind4 stance would have a 10% double strike and 100% offhand.
Garth