When I’ve looked at the cushion again I realised I’ve been over-thinking this.
If I make the cushion 16” the triangles block could be 15”. (Plus an additional narrow border)
There are 5 triangles across the block. Finished width would be 3” each.
There are 4 rows so the triangles will be 3.75” high.
I should be able to draw the triangles.
