bearisgray

When making HST's, the theoretical amount to add to the the "finished size" of the cut square is 7/8 inch. Adding one inch works a lot better for me.

They usually end up "really close" to what I want when I add that extra 1/8 inch - and they almost always end up "skimpy" when I add only 7/8 inch.

When making QST's - the theoretical amount to add to the "finished size" of the cut square is 1-1/4 inches.

Adding 1-1/2 inches works better for me when cutting a square into fourths than using the exact 1-1/4 inch theoretical amount to add.

If you cut the 9-1/4 inch squares (what the pattern says) to 9-1/2 inch squares and then trim down the finished units, I think it will turn out better. And then trim the quarter square units like many trim the HSt units.

My theory why I end up with "skimpy" units when cutting a square diagonally into halve or quarters -
When I cut around a template - my piece is a teensy bit larger than when I cut a square up - the "extra" when I cut around a template is taken up by the turn of the seam. Or to put it another way - my unit ends up skimpy when I cut up a square because fabric is taken up by the "turn of the seam"

That may be the reason that the squares that you cut at 5.5 instead of 5.25 inches ended up being "perfect".