I have never found homemade stuff to be effective other than for a very short while eg 15 minutes.
But it does depend on how bad your flies are and how much your horse suffers.
2 of mine don't seem to notice but little cob gets 5 tones as many flies on her as everyone else and obviously really feels the bites and cones up in huge lumps.
So I like chemicals! Most effective 'normal' repellent are leo vet Phaser and battles summer fly cream.
For the big guns you need an insecticide. Asorbine ultra (black one, £25 a litre) and tritec 14 (£50 a litre) genuinely work and last for days once they've built up a bit. Note both say 'do not apply to horses', do your research and make sure you are happy to use them.