Being originally from SE PA, I know what you're going through! I'll take the rattle snakes in the desert over flies and mosquitoes, any day!! LOL
Oil based products are the best, even though they attract dirt. I've heard you can brush opposite the lay of the hair, spray the horse liberally and then brush with the lay of the hair. Maybe this helps keep the spray active longer.
Have you tried a roll on which you can massage into the hair well? I use the Endure roll on for faces, ears and sheaths and it lasts about a week.
I don't have experiene wth the fly boots, but I tried the neck and leg bands which have the citronella in them and both horses broke out so badly, I took them off and tossed them. I tried to get my $ back and couldn't get a response from the manufacturer.
BTW, I use the fly spray on myself, too!