FLEX INDEX DOESN'T EXIST. 100 stiffness rating means fuck all.
Don't pay attention to that shit. It's absolutely garbage and doesn't correlate to anything. Try on two different 100 flex options, sometimes from the same brand, and they will flex entirely differently.
This is how you should rate it... if you can easily flex and feel like you're either flexing to the point where your heel is getting added pressure or youre flexing your knee well past the toe of your boot, then it may be too soft (or if you feel like you're out of control when skiing variable terrain at a fast speed)
The Seth Morrisons come stock with a 10 tongue... but you can look on the inside and see where the arrow points to and see which tongue you have. I have a pair of wallisch boots with a 10 tongue, for example.. Because the 6 tongue was hilariously soft.
I feel like the 8 tongue is a pretty happy medium unless you weigh alot or are hyper agro and smash through bumped out shit at mach 10 with your hair catching fire... and usually in that case, the distortion of the lower is usually too soft anyway with FT's... and leads to a bit too much lateral torsion in the heel for some dudes.