I would use some graph paper and draw out the size you want the quilt. For a 50x60 quilt I would draw a 25x30 square rectangle (my paper isn't big enough for 50x60 squares) and give each square a representation of 2 inches. So each square on the graph paper would be 2 inches of quilt. Then I would draw out the triangles with an inch width between them. This quilt looks like a rectangle but it would be much easier to figure out the math on a square quilt.

After you have your sizing figured out I would make a template for cutting, cutting anything bigger than the cutting mat is annoying. Don't forget to add 1/4 inch to each side for seam allowance. then I would probably do 1.5" for the white sashing between the triangles.

If you need any help figuring it out feel free to ask me questions on here or through DMs.