Hi FrogPrince, and welcome to the Thorn Cycling Forum!
Congratulations on your decision to order a (new) Raven -- I can just feel the new-bike excitement in the air and am delighted for you!
I just measured the removable back plate on my Thorn Expedition rear rack. It comes with holes drilled on-center for 90, 85, and 80mm.
To mount my own B&M Toplight Line Plus taillight (50mm centers), I simply used the center hole in the Thorn plate and drilled another next to it, leaving enough room to also mount a Portland Design Works Radbot 1000 LED blinky. For photos, see:
http://www.thorncycles.co.uk/forums/index.php?topic=4523.msg22085#msg22085If you wished, drilling two holes 50mm apart would allow you to center a 50mm light in the upper portion of the bracket. The bracket is designed to accommodate a variety of lights and still allow easy customization by drilling where needed.
Hope this helps. Congratulations and welcome once again! Can't wait to see the photos of your new steed. Oh, waiting is such sweet agony, isn't it?
Best,
Dan.