No that add 7/8 is for making two HST units but you described making your sawtooth star with FG units. (which is the way I usually make them, why put an extra seam in if you don't have to)
For what its worth, I save the cut off bits from the FG and sew those into HST. After trimming them you can make a 1 3/4" hst or trim down to 1.5" HST. I save them up to use them in scrap quilts.