If you go here: https://www.maximpulse.com/permethrin/ivermectin-calculating-a-dose.html and scroll down a ways to this heading "Where did I get those numbers?", he gives detailed info. He also has a video at the top, which I personally found confusing.

I double checked his math and it seems right to me, but feel free to double check.

The liquid, for subcutaneous injection on your horse, is 1% which makes for easier math as it turns out to be 10 mg/ml.

See? Algebra is useful!

I too am looking for this answer.