For HST I do as others do and measure up and trim. I think we are missing the point here however with her question as while there may be some HST in the quilt there are other blocks that use the 1/8" increment measurement. Personally I have not made blocks using 1/8" increment measurements other than I frequently use 3/8" seam allowance on my bindings because that is the width of my presser foot. Typically, quilting and patterns don't use 1/8" increments.
If I were doing this quilt I would more than likely follow the pattern and tediously cut out the pieces as the pattern maker has directed. She probably did this for a reason that isn't readily apparent to all of us. I think if you don't, you will more than likely run into fitting blocks together problems far greater than the aggravation of measuring 1/8". I would mark my ruler with a thin strip of painters tape so I could readily find my measurement especially if a particular measurement was used frequently. It shouldn't be that much more difficult to cut something 3 7/8" than 4 if your lines are clear when measuring. Take your time and enjoy the journey. Think of how much you are going to cherish the quilt when it's done.