I like to oversize and trim after afterwards. I cut 1” bigger than the finished size, sew 1/4” on either side of the diagonal and cut on diagonal line. It looks like you need 18 HST of the smaller HST per block. I would use the 8at a time method plus two. I use the bloc loc ruler to trim. Your problem may be that joining so many HST on the exact 1/4” results in it being smaller when pressed. Try moving your needle 1 position to make a scant 1/4”. Or the other cause could be you are stretching the half HST (setting pieces) since they are on the bias. Nice block.