I print murals for a living and the best method for cutting from a roll is using an auto cutter.
Secondly if auto cutter is out of the question, then using a tape measure and a blade, Put the measure on the top of the box, measure up to a point on your body where the length is , remember that length.
Pull the paper straight up to that point in your body, then let the paper roll back on itself and hang it over the box edge, use a straight edge and cut. There will be no issues and this is how all mural printers cut paper on the fly if they do not have an auto cutter in their room.
If any one here knows where I can find an auto cutter I would like the information, bending down to cut is a PIA for me these days.