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.