Really cute, JustJan!
Marcia, I don't have the ruler you are asking about. Pretty much I cut the rectangle, and then cut the two triangles with my easy angle ruler and sew them on and flip them. Then I trim the corners off the rectangle underneath.
Some patterns give you the size of a square to bisect, to get that big triangle in the goose. Then you can add the small triangles and have nothing to trim (no 'bonus' triangles). I find bonus triangles more trouble than they are worth to find a use for...