Originally Posted by NJ Quilter

Here's my method when squaring up HST's... I take the sewn patch and place on my cutting mat. Line up the 45 degree line on the sewn seam. If the finished size of the HST is to be 2.5 inches say, I slide that 45 degree line on the seam until I get to the 2.5" mark on the ruler with the barest of fabric to be chopped off. Do this for all 4 sides. For two of the sides you will need to flip the ruler to get the 45 line going in the correct direction. By the time you are done all 4 sides your HST should measure an accurate 2.5" (or whatever measurement you are working with). If it's still a little off, follow the same process and trim until accurate.