That's an old, yet very effective, Highpower Rifle trick. Years ago when I shot Highpower I would employ that trick whenever I suspected my front sight focus was substandard. It provided enough of detail on the front sight to allow you to be certain you're focusing on it.
Kevin, no it doesn't matter which way you do it, in fact it doesn't even need to be diagonal, you can make a squiggly line if you like.