Overgrowth only counts unit deaths the champion has direct vision of. The effect radius is reduced when there is terrain or nearsight blocking direct vision on dying units. Shared vision with other allied units observing the dying units is ignored.
PENDING FOR TEST: Interaction with shared vision of an allied champion that too has Overgrowth.
The percentage bonus is split into a Damage mitigated by Master-At-Arms (R)base and Champions killed with Comeuppance (R)bonus health portion.
This was to prevent bonus health scalings from abilities such as Feast from benefitting overly much from his base health being factored in as bonus health, as these two effects created large factors for the stat modifier.
Because of this, Champions killed with Comeuppance (R)base health scalings such as Guardian Angel revive health and Friend of the Forest's health cost are increased by the modifier.
Passive: For every 1 monster or 2 enemy minions that die near you, you generate a stack that each grants Champions killed with Comeuppance (R)3 bonus health, stacking indefinitely. At 30 stacks (30 monsters or 60 minions), your Damage mitigated by Master-At-Arms (R)base and Champions killed with Comeuppance (R)bonus health are increased by 3.5%.