Use the tool in the toolbar with the blue 10 and double headed arrow to mark the calibration length.
Clicking it will cause the dimension marker to pop into the frame. Drag the ends of the arrows to the
points you want to use and type in the known distance at the midpoint. I recommend zooming in with
the magnifying glass to assure accurate placement. (To regain the use of the cursor once you have
zoomed, cycle the magnifying glass to its blank state with neither + nor – marks.) Once the calibration
is finished, you can drag the blue double arrow anywhere you want and measure other things in the
image. The calibration won't change unless you retype the distance information.