Sounds like you on the way to sorting this out. If you're having trouble working out how many segments to expose, you could try marking the paper before exposing with a fat felt tip marker to indicate where you are to move your blocker to to make sure you only do how many you've got space for. Another issue you can have that will confuse is if you manage to bump the paper while covering up a section, although you should notice that the image formed is blurry for that one. If you do manage this, chuck that test strip out and do another as nothing good can be determined from one of those.