Ohio Star is not a very forgiving block for trimming down because you will lose all your star points when you join the blocks or attach sashing. Does your machine have the capability of moving the needle over a bit? If so, you may want to move it over one click to see if this helps with your 1/4" seam allowance. I don't have a die cutter machine so I cut with a rotary cutter. For Ohio Star I will make the hourglass units slightly oversize and trim the units down to the correct size. I don't think you have that option with a die cutter so your only option is to work on getting your seam allowance correct. Another thing that can affect your 1/4" seam allowance is thick thread. Maybe you can try a couple practice pieces using a finer thread to see if that helps.