This feels like a bit of a workaround, but I have (through trial and error) come up with a way to scale a background image to be the actual size in Carbide Create.
I’ll first say that it would be much better if Carbide Create automatically imported background images so that they would be the actual size at a resolution of 300dpi. This is standard for most scanners, and kind of an industry-standard for producing images for print. So for example, if I scan a flat part or a tracing on my regular desktop multifunction printer/ scanner, it produces a file that is 8.5 x 11 at 300dpi. If Create sized the imported background image to 300 DPI, then the scale of the scan would be an exact size, and I could trace and mill and get a properly sized part. For some reason, when the image is imported it scales closer to 25 dpi. I could resize my image to 25.4dpi and get and accurately scaled image, but the loss of detail at 25dpi makes tracing accuracy difficult.
To get an ACCURATELY SIZED background image, I have found that if I open my scan in a photo editor (I use Photoshop) and resample the image to 254 DPI @8.5x11, I can import it into the Create background, and then set the background image scale to 0.1 and get an accurately sized image.
If I were to take a photo of an item, It would be easier to set the scale in photoshop than trying to guess at the appropriate scale factor in Create to get the image to be the correct size.
So this works, but it is a bit of a workaround. The feature request for Create would be to be able to specify the size of the background image (dimensions and dpi), or be able to click 2 points on the image and set the distance in inches or mm and have Create figure out the appropriate scale. Ideally, we would be able to choose between these two options.
In the meantime, use 3rd party software to scale your image to actual size at 254dpi, and set the background image scale in Create to 0.1 and trace away.