Ok, so the spell guidelines say "chill a person so they lose a fatigue level" is base 4, and the higher level begins to do actual damage (lvl 5 +5 dam, lvl 10 +10 dam).
Should higher levels be able to fatigue rather than damage?
If yes, should it be 1 extra Fatigue per magnitude, or per two magnitudes (due to the added complexity of not wanting to cause real damage)
If no, this would be because any further cold would cause frostbite, and thus real damage, right?
I know one could just make do with 1 fatigue level, and then make the Duration Diameter, costing 1 fatigue per round. Those 2 minutes should be more than enough to take most people down.
But IF you allowed the spell to cause more than 1 fatigue at once, this could easily become the easiest and most popular "pacification" spell vs. magi. Since it does no damage, you can get away with it easier. Plus it's a relatively low level spell (considering it could become a one time pacification spell, as compared to a 1 round killer!), you can get a decent penetration.
This seems a bit easy, so I'd consider allowing any Fatigue after the first level to be resisted by a Fatigue Roll vs. some set difficulty (9 perhaps ???).
So the spell would end as:
Incantation of Winter's Drowsiness PeIgXX
(Base 4, +2 Voice)
Causes temperature to drop rapidly, making the target extremely exhausted. First Fatigue level is lost without any resistance roll allowed. Second and subsequent levels may be resisted by a Fatigue Roll of 9+. Warm- or winter clothing may give a slight (+1 or so) bonus to the resistance roll.
Level 10 is 1 Fatigue
Level 15 is 2 Fatigue
Level 20 is 3 Fatigue
Level 25 is 4 Fatigue
Level 30 is 5 Fatigue