I am doing this now and find the two squares, line down the middle etc the easiest way. My finished size of the block is 3.5" so I got the 3.5" Omnigrid ruler and recut.
I started with a 4" sqwuare and while it worked to recut, I then started doing 4 1/4" squares ad it was easier with a bit more fabric to trim. I also found in the piece I am doing that my accuracy is better to press all seams open. At first I presed the HST to the side ad it gave too much bulk at the ends so then started pressing open and much less stress. In the block I have 12 HST and 4 plain squares and time is important and ease of assuring accuracy.