I agree with what has already been said: You are doing things right. The only thing I would add is to baste down the side edges within a quarter inch as you advance the quilt. To keep the sides straight, I use both my vertical channel lock and also a longarm centering tape overtop the quilt. I have also heard of longarmers putting a piece of tape on the belly bar to mark the edges of the quilt top, but I never understood how that works, since the backing is larger and also ontop of the belly bar on my machine. I guess whether or not that works depends on your frame and how each piece is loaded.