I've cut quite a few sheets (thousand or so) from Ilford Express and Ilford MGIV 4 inch and 5 inch wide rolls (500ft) over the years. You may be able to apply the same method to larger rolls. I took a cardboard box and placed a slot the width of the paper on one side. The slot is lined with felt as to not scratch the paper as it passes through. On the other adjacent sides I made a hole in the middle of the box as to support a rod (pvc pipe) that will pass completely thorough the box and roll of paper. It's basically a homemade paper dispenser. I set it near the guillotine cutter and pull the paper out to the stop on the cutter board. I cut off the sheet and continue. This I do under a safelight.