John, Good questions and you'll probably get 50 different answers as most of what you're asking is based on individual tastes. I mask a lot of my images with rubylith that crops the film notches, then float the image with a 3/8" white border around the image. So your window mat would be 3/4" wider and longer than the image. I also mask when coating the paper and leave an area 1/8" outside of the film area (including the film notch area). This will leave a small black border around the image. I then float that in the window mat with the 3/8" white border. All these dimensions are just personal preferences. Some people use 1/2" white border around the image. It all depends on what looks best to you. You can also cut your window mat so it comes to the edge, or just covers the edges of your image or you can even crop the image by how you cut your window mat. Buying already cut window mats eliminates a lot of what you can do. If you have a mat cutter you can experiment. I use a cheap Logan 750 and it cuts mats beautifully. But the bottom line is...what looks and works best for you. I corner mount or hinge mount my platinum prints.