When trimming QSTs and Hourglass blocks, I always start by rotating my block so that the 45-degree line of my ruler is on top of a pieced seam. Then I slide the ruler along that line until all four corners of the block size I'm looking to trim to touch other seam lines. In the case of QSTs, one of the points will float and not touch a seam (the lower right hand corner in the photo above).
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