[mivayan]

Is my understanding of fatigue for communion spells correct?

Fatigue for master and slaves is calculated the same way.

x communion slaves and 1 master. Pseudocode for calculating the fatigue the slave bob gets when the master casts a spell:

fat = spell fatigue / (x+1)
fat = fat + enc -- Bob's encumberance.

For the purpose of calculating fatigue, even the communion slave bob gets this boost, even if he has no magic skill:
2 slaves: +1 to all paths
4 slaves: +2 to all paths
8 slaves: +3 to all paths

if bob's magic skill is 1 more than needed, fat = fat/2
if bob's magic skill is 2 more than needed, fat = fat/3
etc
....
if bob's magic skill is 1 *less* than needed, fat = fat*2
if bob's magic skill is 2 *less* than needed, fat = fat*3
etc

A bit suprising to me that the slave's encumberance is modified by the slave's magic skill.