Jenny Doan takes two equal squares and lays right sides together. She sews a scant 1/4” around the edges and then cuts on the diagonal. On both sides. Makes 4 triangles. Lots of bias though. I’d starch it heavily and only press it, not iron. Don’t know how to figure the size though. Someone here could tell you that. She also makes 8 triangles at a time. Watch her tutes! I know some people have a problem with her, but she has some excellent tips on shortcuts!