If I wanted to develop by inspection 6x12 or 6x17 negatives from a roll, in a tray, is there a way I could cut the individual negatives before developing, and know that I have not cut through an image? I am thinking that the arrow start mark on roll film makes every roll uniformly spaced, but I can't figure out how to measure in total darkness without risking scratching or dusting the film or cutting images.