The way I do them is for instance, take two 5inch blocks & place them face together. Sew 1/4 inch all the way around all four sides. Then cut them from corner to corner both ways & you end up with four perfect 3 inch half sq triangles. I have found what ever size half sq triangle you want, just add two inches to the size of block you sew together. For me they come out perfect every time & all the same size.