As a professional aromatherapist, I would urge to you use far less essential oil in your blend. 1 teaspoon is approx 5ml of oil and there are 100 drops in 5ml. As you are adding 500 drops (25ml) of oil this could be very sensitizing to some animals.
Basically you only need to add 100 - 150 drops of essential oil to 500ml of carrier (incl water, cider vinegar, skin so soft etc). Water has the ability to absorb and transmit molecular and subtle energetic/vibrational information, in other words it magnifies essential oils so you use less than if adding to a cream or base oil to get the same affect.
Essential oils that I find have fly repelling properties are:
Eucalyptus Citriodora (lemon scented)
Eucalyptus Dives (peppermint scented)
Cedarwood Atlas and Himalyan
Neem oil is also very effective and can be added to blends, I generally add 5ml (teaspoon) to 500ml
Stay away from using base oils as a carrier as they make the coat sticky and once the essential oils have evaporated you are left with vegatable oil which attracks the flies and dirt and fries your horse in the sun!