Print Layout

Hi everyone. I am using Corel Draw7 Home and Student to create a large plan, and want to use the scaling options in the print dialogue to print just a small part of the plan. I can change the scale OK in the layout options, then change reposition the image to top left, centre etc. But when I select reposition to Custom, I don't see any way to change the positioning of the image. I'm sure I'm missing something obvious!