Double cheking the digits water man is correct and 57% seems very conclusive
Permafrost shield freezes creatures for three turns, 30% of the time (on average).
The freeze starts just after the attack, and ends just after an attack opportunity.
That means
- a critter misses 3 attacks every time it gets frozen
- a critter gets frozen (on average) 3 times for 10 successful attacks.
10 attacks + 3*3 turns frozen = 19 turns to do 10 turns worth of damage, or 9 of 19 attacks blocked. 9/19 = 47%.
If it had frozen critters before they did their damage, that would be 7 attacks in 16 turns (56% blocked), but it doesn't.