Specific Details on Herbs for Rosacea

I am very excited about your answer to rosacea. In the summer I hope to grow many of these. When making tinctures what part of the herb will I need for each? Oregon Grape root would be the root, however with the others (Yellow Dock, Borage, German Chamomile, Burdock) would I be harvesting roots, leaves, stems, or flowers? Does it matter?
In the meantime I am having difficulty tracking Oregon Grape root. Is there a possible substitute or should I keep searching so that I may have use all the components at the same time?
Is there a way to check the potency of purchased tinctures. Some have said to "take 20 - 30 drops" - that doesn't seem right.

Oregon Grape (Berberis aquifolium) is also known as Mountain grape or Mahonia aquifolia. It grows mainly on the mountainous western coast of the U.S. and southern Canada. It is specific for eczema and psoriasis. Barberry (Berberis vulgaris) (part used: bark of root or stem, collected in early spring or late autumn) is a close substitute.
Other herb parts used: Yellow Dock: root; Borage: leaves collected early summer, just as the plant is coming into flower, on a dry day; German Chamomile: flowers collected in summer; Burdock: root collected in September or October
You are right to question the dosage of purchased tinctures. The potency of tinctures should be on the label, but historically that has not been required. If you purchase tinctures from a good manufacturer, they will be made according to the proper technique: time of collection of herb, alcohol amount and potency, length of time of extraction in alcohol. Using a well made tincture means the difference between a herb working for you and the herb being useless. Ask knowledgeable staff at a health food store for the best brand, or better still, find herb manufacturers recommended in reputable books in your book store or library by herbalist authors Alan Tillotson, Aviva Romm or Donald Yance.
I recommend that you purchase a good, basic herb book, such as David Hoffman's " The Complete Illustrated Holistic Herbal" for guidance in collecting herbs, parts of herbs used, dosages and making tinctures.