Using lighting centered at the same height as the target would be good (lighting down onto the target might cause direct reflection from the angled ruler) and using lighting from 45 degrees to the left of the target would be ideal, to minimize direct reflection from the main target and to avoid the shadows which lighting from the right would cause. So while most any lighting will work and the few configurations that won’t will be obvious when reviewing the shot, a horizontal lightsource quartering from the left would be simplest and most foolproof.