Stamps - measuring angular cutouts: Angles can be measured directly, no calibration needed. Here is a stamp image of the area of interest with left and right cutout: 
Annotations and measurements use a separate layer, the original image will not be modified. First we need to create that new layer by clicking on Layer > New on the menu bar. The defaults are in most cases ok but we should give this layer a meaningful name.
Now the draw icons on the righthand side just below the menu should be no longer grayed out, meaning they are available.
The angle button allows us to match the cutout with two lines. The angle in between will be indicated in rad, a scientific way of measuring an angle. Just multiply by 180/PI or about 57.2958 and you will get the angle in degrees.
Using the text function on the Draw menu you can enter the angle in degrees between both overlay lines. Next it might look better to distinguish measurement lines by color from other help lines and text. On the DRAW icon bar a heavy left tilted up-arrow near the screen center allows you to select an object on your measurement layer. Going to Draw > Properties allows you to change color and the endpoints to arrows for example.
The result should be similar to the images at right. You can save the original image with additional layers in .sft format only. But the Export to Image option in the File menu allows you to export the sum of originial image and layers. With File > Save As you can save then in several standard file formats.
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