PMG

Hi everyone, I am new here, but a frequent user of Paint.net and as I was searching the Help I don't seem to find what I'm looking for. May be because the English terms I use are not proper (I'm not native English speaker). Question is : I am using Paint.Net to move furniture on the map of my future house. How can I set the Zero of the ruler on the corner of a room for example, then anywhere else ? Thanks for help. Meanwhile, I'll look for another help and may be post another question!