are you trying to figure out your pace or where to start so you don't get bottle necked.
My experience in such races is that most people are fairly conservative at the start (although not nearly conservative enough, if that makes any sense), and more than willing to let you pass if you ask. If it were me I would look at my percentile placing in the 50k and seed myself accordingly and feel horribly nervous if I felt I was too far up. Actually if it were me I would start at the end, warm up for 2 or 3k and then very slowly pick my way through.
I would err on the side of starting towards the back of the pack as I know I will run harder if I think I am holding people up. To figure out where in the pack that is I would look at past results to get a sense of where I should be. It is a long day, and you have lots of time to make up for being held up by slower runners at the start.