Flex index aren't directly transferable between brands. Flex 100 on Nordica and Salomon is quite different.
But as far as a rough estimation goes, the previous post speaks the truth. You should also take in to calculation that the tongues has a progressive flex and becomes stiffer the further you flex them. When I switched to three piece boots three three or four years ago (to green/brown konflicts) I chose to go with the standard tongue which were slightly softer in the beginning of the flex than I was used too but the were perfect when flexed.
I now ride Il Moros with the stiff tongue.