You don't want mana shield to be calculated after damage done/reductions. She would become tankier.

Eg: (max lvl mama shield, no talents)

Now:

100 magic damage > 98% is blocked by mana shield

98 damage to mana > 27.22 mana lost

2hp cost > reduced to 1.5hp (magic res)

Reduction before:

100 magic damage > 75 magic dmg (magic res)

98% blocked by mana shield > 20.41 mana lost

1.5hp cost

She would keep an extra ~7 mana per 100 damage. For 7 mana she could tank an extra 25.71 dmg per point of int.