Each item has a potency. A healing cover has potency equal to the two components' potency multiplied together. The potency of a healing cover made of black mushroom (5) and tooth (4) will thus be 5*4 = 20.
5 : Black Mushroom, Gland, Twisted Unicorn Horn, Heart
4 : Green Mushroom, Lovage, Rosemary, Tooth
3 : Blue Mushroom, Camelia, Eye, Nettles, Sage, Yellow Mushroom