I have, say, an approx. 1:2 aspect image.  I want to print to a 4:5 aspect (16” x 20”).  I would like a 0.5” border mat on the sides of the long edge, but do not know the exact space to fill on the short (top and bottom) edges.

Is there a way to do this (using ‘photo mat’?) without manually calculating the space?  That is, automatically fill the open space remaining above/below the image.

In my opinion, it'd be easier to just do the math: add whatever size print you want and then hover over that print on the Live View to display the actual print size. Then add a photo mat that is (16-X)/2 on the sides and (20-Y)/2 on the top/bottom where X and Y are your width and height of the print.  But if you really want to, you can place the mat first and then the print on top of it.  For example,

You could:

- Add a 16x20 "Template" (last thumbnail) to the page
- Click on the 16x20 template on the Live View
- Select the color you want on the "Inner" border and set the inner border to 16 inches

This will give you a 16x20 mat (16x20 rectangle of the color you chose on the page).  Then you could place the print you want to print on top of that mat by making the page freehand, changing to the print size you want, and dragging/dropping the print on top of the mat.  You could center both in the full page editor to align them.

P.S.  The method above uses a technique which allows you to put a rectangle of any color anywhere you want on the page: just add a template and set the border to any value that would cause the border to cover the entire template.  So if your border (on a template) is more than half the size of the shortest side, you get a colored rectangle... more than 4 inches on an 8x10 template, more than 8 inches on a 16x20, and so on.  I just used 16 because that's obviously larger than 8 inches and it's easiest just to enter the dimension of the shortest side.  Also, after placing a print on top of that colored rectangle (mat), it acts like a photo mat.  Meaning that if you click on the mat (not the print on top) and move the mat, the print on top will move with it and the two will move as a group just like a normal photo mat.  But if you click on the print itself, you can move the print on top of the mat to get uneven borders just like moving a placemat with a dish on top will move both together yet if you grab/lift the plate and move it, it'll move on top of the mat.

Mike