If you have problems with the math try this: measure the two sides of your borders (I think you said 4 and a half inches) and draw on a square on a piece of paper using those measurements. Cut the square into half square triangles. Now lay one of the triangles on a new piece of paper and mark all three sides. Measure a quarter of an inch seam allowance on each side and mark the lines. You now have the size of your half square triangles for your corner pieces. Make a sample just to make sure they fit correctly.