theres a difference between how much a legendary contributes and how much healing it gains you though.

like lets say theres 1000 damage going out over the course of a boss fight. i heal 500 of it with wild growth, 300 with rejuv and 200 with lifebloom. thats 50% wild growth, 30% rejuv, and 20% lifebloom. then i put on a legendary that gives me 10% healing. it might account for 10% of that 1000, but in exchange that will lower how much healing my other spells did. so the new distribution might be 45% wild growth, 25% rejuv, 20% lifebloom, and 10% legendary. it changes the percent distribution, but it doesnt make me heal more then 1000 because thats the maximum possible health i can heal over the course of that fight

the way i see it, as long as nobody dies and everyone stays at full health then it doesnt really matter what legendaries you use. i could probably unequip my legendaries and still have the same results