With perfect sewing your center would be 50.5 and the left/right HST borders 51.5". If they are both the same size, you are right, there is no room for a floater. But since the difference only needs to be an inch, it is fairly easy to widen your HST borders a little in several places, by making the seam allowances slightly narrower than 1/4". I would do that in four places across the border to see if you can get the HST 1" longer than your center. Then just use the chart on page 3 to do your calculations.
BTW, my center was 51" and HST borders 52"--nobody is perfect.
Any suggestions? 
