Add 7/8” to measurement of side A. Cut a square this size and cut in half as shown. I always add 1” rather than 7/8”, this gives you ‘jiggle’ room. The square unit formed from the rejoined 2 triangles can be trimmed to the desired size.
Example of side A = 4"
Cut squares 5".
Cut these in half.
Rejoin to form a square and trim to the desired size - in this case trim to 4 1/2" for a 4" finished size
Instead of cutting the square in half you can if you wish draw a line diagonally across the square.
For a 4" finished size I cut 2 squares 5".
Draw a line diagonally across the square on the wrong side of the fabric.
Place 2 squares right sides together, sew down 1/4" each side of the drawn line.
Cut on the drawn line.
This will give you 2 squares made up of 2 triangles.
Trim to the desired size - in this case trim to 4 1/2" for a 4" finished size.