As long as you are working on HST's, let me enable you a bit more. Check
www.blocloc.com & watch the video. No rocking! You can get a ruler that is bigger than the block size you will finish, so get one that is able to be used for your most often made blocks. As you will see in the video, you're going to slide it along the seam line to sliver trim, so a bigger one will work for different sizes. That said, I'd suggest one that isn't too much bigger than what you need - an inch or two is fine, but having a giant one for small blocks is more cumbersome. LOVE them.