I cut a length of paper (joined with sticky tape if necessary) the width of the border, and the same length. Then I fold it in half, quarters, eighths and so on until I have the paper creased into regular sections all along its length. Then I know that any design that will fit in that folded section will fit along the length of the border. No math at all! To deal with the corners, the easiest way is to choose a square design to go in that, then cut the length of paper to go from corner design to corner design, and fold as before. In this case, the sections will fit in between the corner designs, so will still fit. If the border across the top has different length sections from the side border, then you may have to adjust. You could leave a gap between sections, or add another section by folding differently. I hope this all makes sense.