This is how I make my cough syrup.
Start with mullein and 'stirfry' it in a dry pan, add water. I use with it dry wild cherry bark, hyssop, plantain,elderberries, even wild cherries, thyme, sage, a bit of mahogany birch bark, mint and bring to a boil, than simmer for a good while. Strain add about 10:4 honey and 10:3 parts brandy or vodka. I do not have an exact measure really, it varies a bit every year. Can't give you a dose for willowbark, it's an individual thing. You might want to scour some books and see what is the most common dosage. That's what I do, when in doubt. Get 4 o 5 out and see what the common denominator is. I found that the 'Herb book' by John Lust has pretty safe and accurate advice for dosages. Although I have more than a 100 books relating to herbs etc. this is the one, I almost often grab for a fast reference. It has lousy drawings, but is comprehensive in material. Not a coffetable book, but a good one to look things up fast. Scour Michael Moore's website and Henriettes, and you will find solid material there as to tincturing and dosages.
http://www.swsbm.com/HOMEPAGE/HomePage.html
C-M