Boy, I forgot the High school Math !! I haven't done it since I got one of those large setting triangles that makes the math go away.
Here is what Fons & Porter have in their original Quilter's Complete Guide
Finished Block Size x 1.41 - Finished Diagonial
Finished Diagonial + 1.25 - size of square
Fons & Porter Quilter's Complete Guide
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