One way would be to copy the original ('finished' size pieces, without 1/4" seam allowances) at 84%, then add your new 1/4" seam allowances.
12" x .84 = 10.08" That tiny fraction shouldn't make any noticeable difference, unless the block has many, many pieces.
Last edited by Neesie; 07-08-2014 at 01:34 PM.