Ivermectin may not be effective for all mites. So if you want to use it, then know that there is a possibility you may end up retreating with something else if it doesn't get rid of them. There are various dosing recommendations, I will link to a few below. Permethrin or Spinosad are more often recommended, and you do need to clean out and treat the coop also. Permethrin sprays or spinosad (Elector PSP) work well for that, to get in all the nooks and crannies. You have to remove all bedding and nesting material, discard or burn it, spray the coop and let it dry and then put in fresh bedding and nesting materials. And you need to repeat that at least 3 times at weekly intervals. The chemicals kill the bugs but not the eggs (Elector PSP is supposed to work with one treatment, but it's MUCH more expensive) so repeat treatments are needed to kill the eggs that continue to hatch, until you've gotten them all.
Egg withdrawl for ivermectin is often listed as 8 weeks. There is no egg withdrawl with permethrin or spinosad when used as directed on the label.
For standard sized birds ivermectin is usually 4-5 drops topically, for bantams it's 1-2 drops.
Poultry DVM says ivermectin is toxic to birds at amounts greater than 5mg per kg of body weight (1kg = 2.2lbs), so care should be taken to ensure you don't use too much.
Personally, I use permethrin as it's much less likely to be overdosed, and it's known to be effective. Ivermectin is also not recommended as much for internal parasites anymore for the same reasons, less effective and there are better choices (fenbendazole-Safeguard, albendazole-Valbazen).
https://www.elkovet.com/services/blog/poultry-lice-and-mites
https://poultrydvm.com/drugs/ivermectin
https://the-chicken-chick.com/poultry-lice-and-mites-identification/
