There is loads of info on this subject in this forum and the position depends mainly on each car in particular. The heads should be nicely concealed from the casual viewer, but do need a clear sight of the road ahead. They cover an area of about 18" from each head and you must bear in mid LEO goes for your front plate 1st, then headlights, so basically you want the heads positioned to cover these items completly.
Hope it helps,
The Blinder is a top product.