Fly spray---How much is too much?

I have been using some 10%permethrin concentrate to spray my cattle. The type where you pour in about half an ounce and mix it with about 1 pint of water in a spray bottle. I realize that Jul-Sep is the worst for flies, so I can find myself standing out there daily spraying them down alot. Mostly I see house flies, but about 6-8 horseflies on each cow (I only have 3). So how much is too much? These heifers are for my own family's consumption and dont want to eat or feed my family permethrin laced beef at dinner time. 

 

I can sit in my living room and hear the horseflies thumping the picture windows all hours of daylight. I am surrounded by soybeans now. They didnt seem so bad last yr as I was surrounded by corn, but it rained most of last summer. 2014 was a normal rainfall yr, I was surrounded by soybeans, and would have a horsefly escort as I drove my car into the driveway.

 

I just want to apply the right amount without overdoing it. The labels never say and I know I wont totally eliminate the problem with the spray alone. I just dont want them losing blood and calories stomping and swatting flies instead of grazing and napping all day.

Fly spray---How much is too much?