I mount all my prints in pre-cut Pinnacle mounts with a black core and leave an 8mm white border around them (see photo 1 below). I’m often asked how I do this; it’s super simple and saves you hours of work (assuming you cut your own mounts).
Step 1. In the Develop module of Lightroom crop the image in the ratio 1:1.454 (see photo 2)
Step 2. For an A3 print, in the Print module of Lightroom set the print size to 384 x 264mm (see photo 3). This size will give you an 8mm white border around the image.
Step 3. Use Pinnacle pre-cut mounts (A3 Standard White, Black Core are the ones I use). I think the narrow black strip sitting against the 8mm white border (the white is just the unprinted paper) really sets the image off.
Print with an 8mm white border. Details of the mount can be seen at the top.
Crop to the ratio of 1:1.454
For an A3 print set the print size to 384 x 264mm