The reason it's good for 25m is that they have 4x the Hp of 10m Hm ones.
It'll proc 1x at least or 2x if it's early enough.
Forgotten One Health
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10m Norm - 489,665
10m Heroic - 829,940
25m Norm - 1,485,593
25m Heroic - 2,489,520
It did 5.8% one week, 6.8% another and usually in the ballpark of 1-1.5m dmg per Yor'Sahj encounter.