Create a new sketch to project what you want to export to Inkscape.
Project whatever you want to export to Inkscape onto this new sketch.
Right-click this new sketch and choose "Save as DXF"
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In InkScape
Open the DXF. Leave the options alone: Manual Scale with factor = 1
Shift-Ctrl-D to open the Document Properties.
Change the units to mm, add a 1 at the end of scale X then tab out. This forces InkScape to put scale info in the file. Then click "Resize page to drawing or selection."
If the document is still larger than the sketch, it's probably because stroke width is very wide. Set it to 1 mm or smaller. Then resize page to drawing again.
Select the whole object and Shift-Click Red to indicate that we want to cut that outline in K40 Whisperer.