I know that this is a quite old thread and I hope "peterphotography" has solved his Problem in the meantime. But I have to add a few comments, because I had the same Problem (see attachment):
The 645's most common light leak is not at the hinge of the back, but at the lock. The cause is that the lock bolt is scraping over the sealing material and damages it. The mean thing is: The affected frame is always the next one, because the "wrong light" hits the piece of film between the supply reel and the film gate. The "razor sharp edge" is the shadow the supply reel casts. This also explains, why it is not at the same position in every picture: At the film start, wehn the reel is full, it is higher up than at the end.
Unfortunately, I have not come up with a real solution yet. So after many desperate attempts to get this camera light-tight, I switched to a 645 ProTL.