Well, another thing that comes into play is the size of the thread. Connecting Threads piecing thread is quite substantial and Superior Threads "So Fine" is thin, yet they are both considered #50 piecing thread. So when you press the seam, the larger thread will take up more space in the seam than the thinner thread, and this will affect how large of seam -1/4" or scant 1/4" - you need to make.