Pat - your center 1/4 square triangle should measure 3" after cutting, 2 1/2" when finished. I don't try to 'square up' that particular piece before the rest of the piecing. Are you using a large 12 1/2" square? if so, use the 4 3/4" line on your center seam to start squaring. Turn & repeat using that same 4 3/4" line on your acrylic square.
Not sure I answered your question?? Hope so! Keep at it - - - -
Jane