Offsetting 3D lines

Seems to have been asked a few times in the past but what's the deal with offsetting 3D lines (with varying Z values at vertex points)

If I used the move parallel command (in top view) to offset a line by 750mm it only moves 145mm away from the original line and the accudraw changes from top rotation to an angled one.

There must be a command to offset 3D lines correctly now in ABD SS6? Surely?