Here is the formula for Rage point loss:
Rage_Points_Lost = 1200 * Damage_Absorbed / Stack_Max_HP
In our case:
Number of Rage points lost = 1200 * Damage Absorbed / Unit Health of the stack.
Example:
With the basic ability, 50% of damage is absorbed. An orc stack is hit for 1000 damage, and the total health of all creatures in the stack is 1000. We get 1200 * 500 (50% of 1000) / 1000 = 600 Rage points lost.

In version 3.0 of the game, the formula was slightly different:
Rage points lost = 1100 * Damage Absorbed / Unit Health of the stack.
They also changed the percentage of damage absorbed.
3.1 - 3.0

50 - 50
55 - 60
60 - 70
65 - 80.
Honestly, I don't see any changes here that would significantly affect the Rage point drain.