Originally Posted by curlygurl
Keep using the same roll of film for testing. After running it through, you can roll it backwards. Watch for the end of the film going this direction- it is loose, not taped down. Tuck it back in place. When you get to the other end of the film, where the tape is, the film will often have shifted slightly. Simply undo the tape gently and re-apply.
Running only from beginning to end wil make the START mark consistent. But you can also tape down the end of the film and run it backwards- just guess on a start point and nothing will be hurt.