Opening Size
The EXACT opening to be cut into the mount in Millimeters ( MM )
As you have already selected the overall size of the mount,
the sizes you enter here will dictate what the border widths will be.
Notes
Unless you are working an absolute opening which you are certain of It is important the size you enter in here is slightly smaller than your picture / image.
As an example.
If your picture / image is printed on paper which measures 100mm x 100mm and you enter an opening of 100mm x 100mm, the paper which the image is printed on will Fall Through the opening