When spraying, I try to spray the eyes as that is the face fly's target area. Have never seen a problem from it. The more widespread the coverage on each animal, the better control when there are more than one type fly involved. Horn flies usually target the area just behind the shoulder but sometimes feed on the underside of the belly, probably due to shading on the underside. Face flies target the face and others may target another area.
Bottom line, just get the most widespread coverage you can.
Just another 2 cents worth.