Check out the feed store for concentrated LIVESTOCK flyspray. Read the back and be sure it lists that it's okay for horses. It will cost 1/10th as much for the same stuff if it comes with a picture of a cow vs. a picture of a horse
I have that kind for routine use here and one 32ounce highly concentrated bottle lasts me a couple years. For shows, I have the fancier / smells nicer stuff but can't say it really works any better than this inexpensive livestock stuff I have
Thankyou everyone. Mainly I was after the "homemade recipe" because I don't like to use anything with pesticides on my lactating or pregnant mares and I needed some relief for my little guys as well. I'm going to try fly predators this year even though I'm starting a bit late for Florida. I'm skeptical on them working overly well as we are surrounded on all 4 sides by cattle. We'll see. I'm so dreading the stinky fly traps. GRRR... The downside of warm weather