I've never used Ivermectin for chickens.
I use this. It's concentrate, so it makes quite a bit. Directions for dilution are on the bottle, as well as directions for storage. Spray below vent to stomach, and under wings every 7 days until all lice/mites are gone. Make sure it gets to the skin. Spraying on top of feathers will do nothing. Works great.
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You can also mix it stronger and use it for the house/run.
I don't believe in using anything as a preventative. Trying to prevent wild birds from coming into the same area as your chickens is all I would recommend. Using insecticides as a preventative runs the risk of them becoming less effective over time as a means to kill the pests.
Good luck!