Here is some helpful information on setting triangles and how they are computed. I find that some quilters use different terminology and methods, so here is how QuiltRef [App Store link] does it, taken from the about.com's web site.
Quilts with blocks set on point have triangular openings around their outer edges where each diagonal row of blocks ends. Two sizes of triangles are used to fill in the blanks. One size fills in the gaps along the top, bottom and sides of the quilt. Four smaller triangles are used to create the quilt corners.
Corner triangles are half-square triangles, created by cutting a square in half once diagonally to produce two triangles with the straight grain on their short edges. Setting triangles are quarter-square triangles, created by cutting a square in half twice diagonally to produce four triangles with the straight grain on their long edges.
To calculate the size of the two squares, (1) Start with the finished parent block size times 1.4142. (2) For the corner squares, divide this by 2 and add 0.875". For the setting triangle's squares, add 1-1/4" to step 1. Unless you have experience with these setting triangles, it is recommended that you add 1/4" to 1/2"; you can trim off any excess after assembling the blocks and triangles.
For a more detailed discussion on Corner and Setting Triangles, including photos and diagrams, see about.com's web site on the subject.
