@Tasyen Thank you. It's so easy, so obvious ...
I just used Factor 0,01 for more precise control.
Let me write it for others:
For stackable fractional damage reduction with Armor Factor 0,01 you need:
Damage reduction - Armor per instance
10% - 11 Armor
20% - 25 Armor
30% - 42 Armor
40% - 66 Armor
50% - 100 Armor
60% - 150 Armor
70% - 233 Armor
80% - 400 Armor
90% - 900 Armor
@Juveqis I found this 'solution' too. But damage reflection deals 1 damage per every instance of spiked carapace. So for 2 items with it it deals 2 x 1.00 damage and healing 1.00 HP is not enough. What is more, magic resistance makes the attacker lose less than 1.00 HP, so you would actually heal him.

But you should heal the attacking unit. And not in "is attacked" but in "unit recives damage".
But the question remains:
Can damage reflection in spiked carapace be turned off?
Why?
To use the damage reduction only for melee normal attacks. Normal Armor works also for ranged attacks.