Here's your formula:
Take the fatigue that's listed for a spell in the manual. For blade wind this would be 80.
Divide by the number of slaves. If you have 4 slaves, this would be 80/4 = 20 "base fatigue" going to each.
Then for each slave adjust the base fatigue by their relative path level to get the fatigue they actually receive.
If they are lower than spell level, fatigue received = base*(1+levels below)
If they are higher than spell level, fatigue received = base/(1+levels above)
They also of course (well, nearly positive) get their spell casting encumbrance as well.
Blade wind is level 3, and 4 slaves that have no earth magic have relative earth path level 2 (0+log base2 of 4, as there are 4 slaves). That means they are 1 level low, so double the base of 20 and give each 40 + encumbrance.
Hope that helps.
And by the way, I couldn't help but laugh when you suggested equipping slave collars. I'd reread the description before you try that
.
(you're looking for a slave matrix which is S1E1 and a greater magic item I think).