I just looked up fly predators and I kind of don't think they would work here. We have black flies and if your horse is white, you can see them on their faces when they are out in the pasture. The kind of flies that these predators are useful for are the ones that hang around the manure pile and barn. Last summer, our first few months here, we had no manure pile and the horses weren't kept in the barn so I am assuming that the millions (well it seemed like millions) of flies on their faces were coming from the woods and the stream down at the bottom of the pasture. So this year, I am going to do two things, barn during the heat of the day and LONG fringe on their fly masks
I use fly predators too. Your right they won't do any good on flys being hatched off of your property. I would say you are on the right track with putting them in a shelter (the darker the better) and using fly sheets and masks.