I am using a small program "Photoelf" which has a very fast cropping function. You can set a window size that can be moved over the page with the mouse and then its one click to crop, save and bring up the next image in your folder. I use this technique regularly to crop out page numbers when scanning books for OCR. So I do a rough crop with the Plustek and a fine crop with Photoelf.
www.photoelf.com