Calibrating floorplan

[illo77]

Hi,

I'd like to know how can I put a floorplan in scale...

 

Basically, I import the original floorplan in jpg and I need to put it in scale before tracing it with the tools of the software...

 

I know there is the option to scale it (1:100, 1:50 and so on) but sometimes I have some floorplans that are not in scale and to put in in scale i should measure the distance between 2 walls (Usually i take the distance manually with the laser). In this way the software should give me back the exact dimensions of the areas of the apartment.

How can I add a certain dimension on the floorplan?

Thanks