Whatever the formula, I always cut these setting triangles a little bigger and then trim them to the exact size after I have pressed them. I'm not the most accurate quilter and this way I get reasonable results.
And Margo forgot to add: Starch your fabric before cutting it into triangles if you want perfection.
From the edge of Sherwood Forest, home of Robin Hood
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I cut my setting triangles with the straight of grain on the long side of the triangle which means I start with a square, and cut in diagonally with an "X".
To know what size square to start with, I add 1-1/4" to the longest FINISHED side of the triangle to get the cut length of the square.
Here is a handy chart to determine the size square to start with: