How does Self-Care healing Speed work?

I've been thinking of running a build around Self-Care that is trying to undo the healing speed downside but I'm a little confused as to how it works? Is the "50% of normal healing speed" multiplicative or additive like movement speed.

Is it...

Additive = Base Healing Speed + Botany Knowledge - Self-Care l 0.83 = 1 + 0.33 - 0.5

Multiplicative = Self Care(Base Healing Speed + Botany Knowledge) l 0.66~ = 0.5(1 + 0.33)

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