A. Every species/cultivar responds to hormone treatment in different ways. Ideally, every plant has an optimum hormone concentration range.(1)
Because of the numerous species of plants, it would be impossible to determine a rooting hormone concentration curve for all plants. Some general guidelines for ratios are 1 in 5 for hard woods, 1 in 10 for medium woods and 1 in 20 for softwoods and succulents. The point is to find a good median range for the largest amount of plants being propagated, usually the 1 in 10 range.(1) Too strong of a concentration may burn cuttings. If in doubt, a more diluent solution is best.
For a reference chart of parts-per-million see Parts-Per-Million Sheet in the technical reference section.
(1) Dirr, Michael and Heuser, Charles. The Reference Manual Of Woody Plant Propagation. Athens, GA: Varsity Press, 1987.