After a bit of trial and error and good old freezer paper, I have worked out the size of the triangle fillers and have started squaring up the picnic quilt. Then after cutting several triangles, I realised I could just cut a square and then cut that diagonally. Better late than never.
My next dilemma is whether to fill the four corners with the orange or the more colourful fabric. I am leaning towards staying with the orange. I plan to use the more colourful fabric for binding - at least, I think that's what I was thinking when I ordered more of it! Oh well, the NZ$ is strong. (I should probably say that the US$ is weak.)