Baby Fence Rail - Pt III

I have been chain piecing the 48 squares into pairs and pressing them.  I had experimented with chain piecing when constructing the piano key border for the log-cabin quilt and I think it is the best thing since the invention of the lock-stitch machine!  It's hard to believe how much such a simple technique speeds up the piecing process.  I use a dry iron to set the seams and then I snip the units apart before "pressing to the dark side"

The 48 squares make 24 pairs which, in their turn, will be sewn together creating 12 four patch squares.

Here I have managed to sew together 12 of the pairs to produce six squares.  I stopped here because the light was starting to fail (to be fair it hardly got light today) and I wanted to get some pictures taken.  I used a couple or three pins to make sure that the seams all stayed matched at the centre of the blocks.  They've come out well so far and the 201K has behaved faultlessly.  The hinged presser foot has managed to glide over the pressed seams and pins beautifully.

I may carry on and sew the remaining pairs into squares this evening but I will need to spend some time reading up on "blocking".  The larger squares should measure approximately 12 and a half inches and finish at 12.  The ones I have completed aren't far out but I really want to be as accurate as possible.  From what I can tell blocking will help me establish the 'true' sewing line for when I piece the units together in rows.  One method advocates using a square ruler and a pencil to establish this line the other a rotary cutter to trim blocks down to size.  I think am leaning toward the drawing rather than the cutting method.  Less scope for disaster at this stage perhaps?