I actually use a script instead of the focus guide, which can sometimes be difficult to position correctly. The script tells you the distance between the current camera and any selected actor, so you can set the correct focus distance without using the guide. (Python Scripts window --> Render / IO --> Calc DoF Focal Distance).

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)