It is very easy to establish the size needed by knitting one triangle from a point until it is the width you want.
I don't remember now exactly how I made this. It seems that some calculation would be needed to determine the positioning of the stripes.
Because the triangles slope in opposite directions on back and front the joins between the blue and white would have been different. On one side the increases for the new triangle could be formed by picking up stitches from the existing triangle. On the other side those stitches would be picked up on a wrong side row so may not look the same.