It's painstaking work to get that focus guide exact on where you want it; it's not like you can switch cameras to see it from another angle. I've read one tutorial on it, but it doesn't solve that problem.

Try using a flattened cube primitive and setting it close to where you want your focus, and then tweak the focus distance until the crosshair appears just in front of it.

And you're right, f-stop doesn't change that distance, it only changes how far things blur out of focus from that distance. Lower numbers will give you more blur, higher numbers will keep things in focus further before and after that point.