I think the colour of the horse plays a huge part.
Mine is a few spot, so an off white with a few spots. He doesn't get that many flies on him. The black mare over the track from him is absolutely covered in them all the time, bless her.
I do feed apple cider vinegar which they say deters flies as it alters the PH of the blood. Am not sure, but it doesn't do him any harm and for a pale horse he has a very gleamy coat.
I did make my own fly recipe - similar to above and it was pants.
The two I rate are Effol, but better than that is the Powerphase.
Interestingly - if you are as sad as me - Bucas did a study - think someone on here cut and pasted it??
Zebra stripes (which their new rug is) attracted the least amount of flies, grey/white the second least, bay/chestnut the third least and black the most.
They are of course producing a zebra stripe fly rug .......... but equally, Tobes attracts less flies than his darker counterparts.