Your Damage Stat as the name suggests only affects the damage done on the opposing target.
Your Incoming and Outgoing Boosts is what affects your Healing Also note that Positive Charms such as Guiding Light and Negative Charms (Wards?) can also affect your healing, either increasing it or decreasing it.
Also note that your critical rating impacts both damage done and healing done as you can critical on both (,) these types of spells.
as the name suggests only affects the damage done on the opposing target.
