We like to determine the doses of herbs (especially tinctures) by starting each person with a small amount and increasing only as necessary. Then we jot down the amount so as to know how much to use next time. Everybody is unique so it is important to look at each person individually with herbs. We each have a different constitution and various deficiencies.
Home herbal remedies are just that. There are some herbs that should be treated with caution when used internally, such as wormwood and rue. But for the most part, there is room for trial and error. The biggest dosage difference is between tinctures and teas. Tinctures are much more potent than teas and tincture doses are more critical. Teas can usually be taken as often as desired.
The Green Pharmacy Herbal Handbook, Herbal Antibiotics, Practical Herbalism, and Nutritional Herbology all contain recommend dosages with their remedies. These were helpful in leading us to come up with dosages for our own creations. Herbal Antibiotics and The ABC Herbal have helpful information on determining dosages for children.