Don't forget that the shielded unit could be on low mana... then it's stupid when the manashield absorbs 6 damage and gives 150 damage back..you should calculate the damage done like this:
damageReturn = min( DamageDone, (Level of Mana Shield for (Damaged Unit) + 1 ) *0,5 * (Mana of Damaged unit) )
(If you are using the standard mana shield MP:HP ratios of 1/1,5/2. Otherwise you should adjust the formula.