The measurement of therapeutic dose remains a mystery. Everyone has an opinion,but Poultry Science Association journals are not accessible to those who are not members. This is from Parasitipedia's site:
http://parasitipedia.net/index.php?option=com_content&view=article&id=2449&Itemid=2716
On poultry, ivermectin at the therapeutic dose is also ineffective against bloodsucking poultry mites such as red poultry mites (Dermanyssus gallinae), northern fowl mites (Ornithonyssus sylviarum), and tropical fowl mites (Ornithonyssus bursa), as well as against soft ticks (e.g. Argas spp, Ornithodorus spp, Otobius spp, etc.)
I once used Ivermectin type pour on ( Eprinomectin aka Eprinex) as well as injectible (Ivomec) made by Merial. It was a waste of money and the scaly leg continued despite treating coops on a regular basis. Topical uses of sulfur based ointments, pine oil were effective in removing Knemidocoptes mites. The key is treating legs often enough and massaging the oils into the scales of the entire leg and toes. I use nitrile gloves and massage up and down the leg manipulating the leg to get oils under scales. Nu-stock is a very good treatment for scaly leg:
http://www.nustock.com/ I go in once a month and use an emulsified concentrate of Ravap, or Permectrin II, or Atroban and treat all sides of the roosts. I find it interesting that i've have had to treat some breeds more than others. My Black Australorp hen has never needed treatment for scaly leg and is over 9 years old.