The perk descriptions of Strength in Shadows & Self Care are very misleading
Per this guide by Peanits, Dead by Daylight math for action speed works as follows: first, positive effects are added to the base amount of charges per second, then the product of 1-negative effects is multiplied by the positive effects. So one reading the perk descriptions of Self Care and Strength in Shadows would assume that these perks set the base self heal speed to 0.35 and 0.70 charges per second respectively, and the synergy with say Botany Knowledge can be calculated by adding 0.5 to them, as there are no "negative effects" in play. That would mean Self Care+Botany for example would have a heal speed of 16s/0.85c/s=~21 second heal, and Strength in Shadows would be 16s/1.2c/s=~13s heal. This is not what actually happens though - Self Care+Botany actually takes ~30s for a heal, and Strength in Shadows+Botany ~15s. The reason for this is because these Self Healing perk are actually coded as negative modifiers to self healing, meaning 65% and 30% healing speed penalties have to be multiplied in after positive effects are added up. The base healing speed remains 1c/s, so positive effects (Base+Botany) becomes 1.5c/s, but then a 65% healing penalty is multiplied in for Self Care, yielding (1 + 0.5) * (1 - 0.65) = 0.525c/s, which is how we arrive at the 30 second heal. With Strength in Shadows it's a 30% negative multiplier, yielding (1 + 0.5) * (1 - 0.30) = 1.05c/s, which is a 15 second heal.
The way the perk needs to be described is the same as how Medkits currently are:
Unlocks the Self-Heal action: Reduced Personal Healing speed of -33 %.
Even though Self Care and Strength in Shadows' perk descriptions don't say it, this is actually what they are doing too. They unlock Self-Heal, which is 1c/s, then apply multiplicative negative penalties of 65% and 30%. This makes a huge difference in how healing perk synergies are calculated and for the sake of consistency and clarity, the perk descriptions should be updated to indicate how the calculations are actually occurring.