I sit normal distance, 2ft probably. Just your run of the mill computer desk.
It's the 23" model so it's not as if I'm sat too close. However, I've now read that this monitor needs to be at a 20 degree angle for 3D to work (basically you tilt the monitor back so the top is 3" further away than the bottom).
I had the same experience with the 27" DM2780D and returned it for a refund. I won't touch another LG, they have by far the worst 3D computer monitors! None of the active 3D monitors I tried had ghosting this severe and the passive 3D is supposed to have less crosstalk.