No, you have to call down the fire mission in the next turn, so its only 1st and 2nd turn. In turn 1 Crenshaw (or other channelling officer) declares the FM, people, that can add chits - do so (valkyries are sooo great for this, as you have at least 5 channellers activated in a single activation - so you don't give your enemy too much time to evac the beaten zone and add at least 5 chits). On the 2nd turn you pray for winning initiative, activate Crenshaw and bring down the judgement on everybody stupid enough to still be near the marker.
So:
Turn #1
activation #n: crenshaw declares FM
enemy activation #n: somebody flees the area
activation #n+1: 5 valkyries add chits, each one for one action
enemy activation #n+1: somebody flees the area
Turn #2
pray, pray again, roll for initiative, win
activation #1: crenshaw brings down the Judgement, enemy troops stupid enough not to flee die
It is best to put the FM marker near already activated units, so that they can't flee.