I personally prefer to allow 1" over, for a 1/2 square triangle, then square it up. But for a 1/4 square triangle you allow 1 1/4, (I personally round up to 1 1/2), cut your first 1/2 sq. triangle, cut, press open, put opposite colors together and then mark,sew for your next 1/2 sq. triangle, cut, press, you now have a quarter square triangle, then square up to the desired size.