I/ve tried paper piecing just using scraps and found it a little hard with odd shaped pieces to get all the seams sewn properly. On this quilt I measured the height and length of the longest let of the triangles and added 1/4" around (roughly 3 1/2 x 2 1/2?, can't remember the height as I did it 2 days ago, that's why I took notes while I was trying to figure it out)then cut strips then cut rectangles using those measurements. Haven't gotten too far yet because I realized I hadn't bought enough fabric for the main color of my square. I'm hoping this works because this is only the 2nd paper piecing work I have done.
I/ don't know too much about rulers I only have straight and various square ones.
Hope this helps