Originally Posted by qbquilts

Since you want the finished size to be 3/4 or 75% of the original size (18/24 = 3/4 or 75%), you need to resize each piece to be 75% of the original size. Like a PP said, subtract the seam allowance first, resize each dimension of the piece, and then add back the seam allowance.

For example: To take a 12" 9-patch block to a 9" 9-patch block. 9"/12" = 3/4 or 75%. Originally, you cut 9 squares 4 1/2" to make the 12" block.

4 1/2" - 1/2" (seam allowance) = 4" (original finished size)
.75 * 4" = 3" (new finished size)
3" + 1/2" (seam allowance) = 3 1/2" (new cut size).

This formula works whether shrinking or enlarging a pattern piece. Just make sure to do each dimension by the same percentage.