You will still need to measure if you cut them and then sew. Actually, you will be making two at a time when you mark your line and then sew. For this I would mark one diagonal line then line up your foot and sew 1/4 inch from the line. Hopefully you have a 1/4 inch foot for this project. Trying to aim for accurate triangles would save you more time than having to cut down each one. (IMHO) Good luck on your project
