I have been tinkering with my Defiler's corruptions for the last week or so, collected some data and done some math and I thought I'd like to share my findings. In this post, I'll explain what factors affect our healing numbers, and how to make those numbers as big as possible with a given number of corruption trait slots.

How much a healing skill heals is calculated as:

heal=(base heal)*(1 + mastery bonus + other bonuses)*(1 + incoming healing bonus).

Here "base heal" is a number independent of character stats; for example, for Fell Restoration, it is 6685. For the HoTs, there is a range of possible values, which can be turned into a range of possible actual heals using the above formula. The following bonuses depend on the healer's stats except for the inc healing bonus, which depends on the receiving creep's rank (and buffs and debuffs from skills including Fungal Spores); these bonuses are given in percentages and inserted into the formula in decimal form (e.g. 12.3% -> 0.123).

On to the bonuses. The part "(base heal)*(1 + mastery bonus + other bonuses)" gives the numbers you see in skill tooltips. Mastery bonus is the percentage number displayed in the tooltip when you hover your mouse above the tactical mastery stat in the character window. Other bonuses include an intrinsic class bonus, a rank bonus, and the 10% bonus from Enhanced Slime. They also include the bonuses from damage-for-X corruptions: percentages mentioned in those traits modify not only our damage, but also healing. Moreover, as you can see in the tactical mastery tooltip, mastery provides way less bonus for healing than it does for damage; relatively, the damage-for-X traits actually increase our healing more than they increase our damage!

There's one more thing: heals can also crit and increase in magnitude this way. Critical rating both increases the chances of a critical heal (capping at 30%, with the 3-corruption set bonus of 5%) and their magnitude. For critical heals, the heal should be multiplied by (1 + crit magnitude), where

crit magnitude = 50% + (bonus from crit rating)

where the bonus is the %-number displayed when hovering your mouse over the crit rating stat. Note: heals cannot devastate (I'm assuming that the devastate chance for heals is 0% and the "normal" crit chance is the same as for attacks, although I have not actually tested this).

Now the number giving the average outgoing heal is

expected heal = heal*[(1 - crit chance) + (crit chance)*(1 + crit magnitude)].

To maximize outgoing healing means maximizing this. The exact ideal corruption setup depends on the defiler's rank, racial traits and other bonuses, like those from outposts. For my rank 7 defiler with all the relevant racials unlocked, assuming buffs from 2 outposts, the best build for 6 traits and without sacrificing health for damage is

power for damage rank 2,
1 x mastery,
4 x critical rating.

This is what I'm running with at the moment, with 3 of each type of mitigations for survivability. Generally speaking, damage-for-X rank 2 is better than mastery, and so are crits up to 4 (after this point, the crit chance no longer increases and the growth in magnitude isn't worth the trait slot). The best class cannon build seems to be

power for damage ranks 1 and 2,
damage for health ranks 1 and 2,
4 x mastery,
4 x critical rating.

Theoretically, this gives me around 12% more healing than the 6 trait set up above.

A final note on damage vs healing: most effects that increase damage do not actually increase healing. This includes the audacity damage bonus and the 5% mastery set bonus. Damage-for-X-corruptions are the exception to the rule.


...well that was a wall of text.
tl;dr: Damage-for-X-corruptions are a great way to increase your healing, and crit is usually better than mastery.