Why not take your boots with you when you go buy them. That way you can try the various tongues and see what works for you.
Generally a FT 10 Tongue is about a 110 flex. 8 = 90, 6 = 70, 4 =50 and 2 =30, how any one skis in a 2-6 amazes me, but i am 5'11 and 170lbs. These are just rough figures and boot flexes never really line up perfectly between any brands.
The most popular selling Flex in most brands is 100-120, now thats all boots not just freestyle, but I would generally say unless your a skinny kid, look at the 8 or 10, or 6 if you like really soft boots. But its all personal, so take your boots and try them out.