The way I create my arch is by asking the client to look straight ahead.
Then, with a pencil, I hold this against the outside of the nose at the bottom, where the nostril flares. I hold the pencil straight up and to the edge of the corner of the eye...any hair towards the middle from the pencil should come out.
The , to get the arch, I hold the pencil/thin spatula against the corner of the nostril and direct it across the eye at a slant so that the pencil/thin spatula goes right across the eye at the same point as the black part of the pupil, where the pencil/spatula touches the brow should be the highest point of the arch of the brow.
To determine the end of the brow, I hold the pencil/spatula against the nostril and slant across the cheekbone to the very outside corner of the eye, any brow hair on the outside of this should be removed.
After I have worked out those areas, I wax out a natural arched line across the underside of the brow.
I have tried to draw a diagram, but it's pants, so I hope you can get the gist of what I'm trying to describe!