I know they have found lower dosages work better for people, but not find much solid research on dosing for dogs for each disorder so I'd start low. However, I am impressed with the amt of vets endorsing it and studies.
http://www.dogcancerblog.com/blog/more-melatonin-and-dog-cancer/
leading to this published on help with cancer survival: https://www.ncbi.nlm.nih.gov/pubmed/10674014
Dr Dodman is a rock star: http://www.whole-dog-journal.com/issues/3_5/features/Supplements-For-Anxious-Dogs_5095-1.html
https://cvm.ncsu.edu/new-research-study-focuses-on-mysterious-eye-disease-in-dogs/
https://www.vetinfo.com/melatonin-supplements-for-dogs.html
http://vcs.vetmed.wsu.edu/research/clinical-studies/melatonin