This is difficult without undertaking some development work . It is not clear if this is for animals or animals but assuming the former one would need to administer the the drug in its formulation to an animals skin preferable a mini pig which has similar skin propertied, and then measure plasma levels . A comparison with the normal route of administration (AUC) will then provide a measure of bioavailability and thus calculate the dose . There is also the possibility of using skin samples from humans to calculate the diffusion through the skin ( Franz cells) but this is an isolated system and will not provide a reasonable answer . Unfortunately the drug and formulation can vary considerably and values from

Thank you

For a new molecule, it is interesting to see first, a mathematical modeling for example with the equation of Pott and Guy (there are others but which do not provide more information) All the mathematical models are absolutely false, but they first allow us to verify whether the molecule has the potential to cross the skin barrier or if it is very limited. If your drug has a log P between 1 and 4 and a MW of less than 500, the amount absorbed may be sufficient. A peptide of high molecular weight will not cross the skin barrier, or so weakly that it will be of no benefit. You already have an idea if the molecule will be able to cross the barrier (the melting point, if it is a salified form, the formulation are other factors to take into consideration) If your molecule has a potential absorption, you can test in 2nd on "franz cells" human or porcine skin, otherwise it is a waste of time.