Animals vary dramatically in size and shape. One species of the same age can be very different from another. The most effective and safest way to dose animal is by weight. After all, we want to adjust their dosages by how much their metabolism can handle and the surest way to do so is by weight. Simply make a fraction and put the animal's weight over 150 pounds. Look at the following examples and I am sure they will clarify what I am saying.
If a dog's weight is 10 pounds, then you would give the dog 1/15 of the adult dosage. If a horse's weight is 1500 pounds, then you would give the horse 10 times the adult dosage.References: