I think you need to compare rows before adding the horizontal sashing. I would do it in sets of 4,(2 top, sashing+ 2 bottom) make sure the new blocks of 4 blocks + sashing were the same size, then connect the extra 2 in the middle. Squaring up before you add sashing is a big pain, but it does improve accuracy. That way if there are any little inconsistencies in seam widths, they are dealt with before they get too noticeable.