I'm probably killing my horses but I make my own concoction as well. It's several herbal oils. Citronella, peppermint, and tea tree. I also add mostly white vinegar 2 C. And water to which I add 1/4 c. Of Witch Hazel (it helps along with the tea tree to keep itching soothed). I buy the concentrated permethrin about 1/4 tsp. And about 1\4 tsp. Sawyer Jungle Juice. All this goes in a 32 oz spray bottle that I top off with the water. You have to shake it during sprays but it will drop a fly and I never see more than one or two flies on any of the horses. In case your wondering, it's the Jungle Juice that is the kicker. It's 98% deet and I've only found it at camping/hiking supply stores. I give a feed through fly off stuff (smart bugoff) and keep the poo raked and spread as much as I can. I used Fly Predators for many years and decided we would test out the feed through's this year. I must say I'm impressed so far.
We used Horse and Pony fly spray and it works really well but it's oil based and if you plan to ride is a mess. It's great for just a stand around in the pasture spray though. I've been shocked at the prices of fly spray lately so I decided to make my own with 4 horses and one with fly allergies. Not sure how much it's costing per bottle but I'm sure it's in the less than $1 range.