This may be easier to do than describe.

Friends of mine cut down a roll of Ilfochrome (24"?). And we shared the purchase of a 30" roll of Ilford Multigrade IV RC, which we have cut down part.

We don't have a 30" cutter, but my friends have a 24" Rotatrim.

They put together a bracket using a broom handle that attaches to the end of a table. On the Table they attached a mat cutter with a built in straight edge, and temporarily attached edges as guides to the paper. In addition, they attached a movable end marker, that clamps to the side guides.

The broom handle goes through the middle of the roll, which turns. The paper rolls of the roll, between the side guides and then under the flip-up straight edge of the mat cutter. The leading edge is pulled through between the side guides until it buts up against the end.

The flip up straight edge is used to guide a Logan cutter for the edge.

The mat cutter is used for the 30" roll. The 24" Rotatrim can be used instead for the 24" roll.

Practice with a cheap roll of wrapping paper first!