Scaling Units to match an imported picture

So, I import a reference picture and I'm going to trace over it. How can I change the units to match reference picture?

Example: I import a 3-view drawing of an airplane. I'm going to design a model of this airplane. I know the wingspan is going to be 85". I need to set the CAD units to 85" from wingtip to wingtip, with the imported picture for reference.

Can I do this in LibreCAD?

In DesignCAD3K I had a "Units" function where I would set two points and then input the distance between the two points. This was very handy. If for example, I found that the location of some parts were not at an intuitive location or of an odd non-standard size, I could fudge the tracing to make a more useful CAD drawing for building the airplane.

Gary
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You have to match the picture size to the desired units. By default the picture is imported with 1 unit per pixel. You can adjust this during the import process in the 'tools options' bar. So first import it with default setting and measure the resulting wing span. Calculate the needed resizing factor and scale the picture with the scaling command or import it again with that factor.