For what it's worth, there are small arrows on both ends of the corner lights that point in the direction the light shines. They are small but have helped me with my interior lighting.

This is a lot easier to see though!

Mods like this wouldn't be necessary if Keen wasn't as useless as mammaries on a zombie when it comes to usability. While the orientation of the block and subsequently the direction of the light offset _can_ be demystified even in vanilla, it, one, requires enabling the butt-ugly "Rotation hints" in Options > Game, and two, requires the knowledge that the offset goes in the direction away from the orientation hint cube's side that is tinted blue if you squint really hard.

Thank you Ace, this mod has helped me improve the lighting of my large creative build significantly now that I know which way the corner lights are facing. Now, back to work for me.