Okay so this becomes more evident when using Healing Nova. Essentially the replicated healing from healing done by Ivy is 1 Replication Rate (just the 35%<52.5%) higher for Ivy than it is for allies, and it seems to have a unique interaction with Healing Nova (or maybe with multi-person (more than just Ivy & an ally) healing items, the only other potential one that I've thought of is the Spirit Lifesteal on Spirit Shredder, but I haven't tested that).
Essentially, the more people that receive Ivy's Healing Nova, the more healing the Watcher's Covenant will replicate. For example, even with just 1 tether, if 4 people receive the Healing Nova (regardless of if they are missing hp or not), then Ivy will receive 4 Replication Rates worth of healing, and the tethered ally will receive 3.
The equations I've arrived at are
I = HN*RR*P
and
A = HN*RR*(P-1)
Where:
I = Ivy's Watcher's Covenant healing received
A = Ally's Watcher's Covenant healing received
HN = Healing Nova healing amount (note that there is also a bug which has some sort of variance to the amount of healing Healing Nova does. For example, even at level 0, where it should be doing 325 healing, I've only had it do 319 or 337. When I level up a boon, it does go to 325, which makes sense as 319 & 337 are both within differences of factors of 6 (325-6 & 325+6+6), the boon scaling of the item).
RR = Replication Rate (over 100, so 35% is .35)
P = Total # of people, including Ivy.
So the amount of healing Ivy will receive from 2 total people is the same as the ally will receive from 3 total people. I made a spreadsheet with data points I tested, both with just 1 tether & with max tether attached.
Another Bug to Note: Once attached, healing is replicated to the last target tethered, regardless of if the tether broke. One needs to be in tether range for the lifesteal & fire rate, but the healing will continue no matter how far. So if an ivy last tethered a McGinnis, and the McGinnis placed a heal on the ivy, the McGinnis would receive the replicated healing, even if the McGinnis teleported across the map
