The formula for HSTs is add 7/8" to the finished size you want for cutting the squares. I cut mine at 1" over to give a little space for corrections and trimming. Some prefer 1 1/4". I bet your pattern has a typo, unfortunately, not uncommon. So, on the quilt I just started, I want 4" (finished) HST units, so I cut 5" squares. I will trim to 4.5" for 4" units in the finished quilt.