The best part about Armor Crush is its flexibility.
There's no loss of dps if you use it under 40s left of Fuuton.

Like I said before, it's good to keep that buffer of 20-30s for fight mechanics or any mess-ups.
You can also let it drain a bit more and get AE in if you're in a burn phase or you know the fight well enough to be sure you'll have time to extend it.

So I don't think we should make hard and fast rules about when to extend Fuuton.
That doesn't give value to Armor Crush's excellent flexibility.
Anytime after the 40s mark is acceptable, and you can adjust based on the fight and even more importantly - buffs.

Obviously, if you do it over 40s then you won't get the full 30s it's meant to give you.
So I guess it goes without saying not to do it if you have over 40s left. But I said it anyway!