I googled setting triangles formula and got this:
Quilt blocks set "on point" are sewn in diagonal rows beginning in a corner (the top left corner). The first row will be one block with the second row having three blocks, each row is completed with a setting triangle Two blocks can be used in this first row, if you prefer, with the second row having four blocks, the third row six blocks, etc. As you see, whichever setting you prefer, the number of blocks in the diagonal rows increases by two in each row. Add rows until you reach the width you need. The rows will decrease by two as you complete the top. Each row is completed on both ends with a setting triangle.
To determine the size of this triangle, there is a simple formula that works no matter what size the quilt block.
The formula: Size of the block times 1.414, then add 1 1/4 inches.
For a 6 inch block: 6 x 1.414 = 8.484 or 8 1/2 inches. Add the 1 1/4 for seam allowances = 9 3/4 inches. This would be the size of a square block needed to cut setting triangles for a 6 inch block. Use the same formula for your size block.
Cut a strip WOF the measurement you just figured, cut squares that same measurement. Subcut with an X. That will give 4 setting triangles from each square. Mark the outer edges of these triangles. Two of these outer edges are straight of grain and and the other two are cross grain.
Read more at Suite101: Quilt Construction: Taking the Mystery Out of Setting Triangles
http://www.suite101.com/article.cfm/...#ixzz0q5eufUxJ
Is this what you meant?