Here is the formula I learned from Linda Ballard. Of course you put 2 squares together!
make 2 squares following this formula. Use the size of finished HST + 1" X 2 + 1/4". Put the 2 squares together. Draw fine lines corner to corner, another line down the center lengthwise and another across the center horizontally. Sew 1/4" on each side of the DIAGONAL lines. After stitching then cut on all drawn lines. Press, open, press and square up if necessary. I think adding the 1/4" gives some wiggle room.
i.e for 2" finished hst. 2" +1" X 2 + 1/4" = 6 1/4" squares