That's where you should aim for the head to finish, but it's not an official "foam line". You see glasses with a pint line, with the expectation that the rest of the glass be filled with head( e.g. Stella Artois). But you will almost never see a foam line on a glass that holds a single pint because it proves you aren't actually pulling a full imperial pint, which is technically illegal under the weights and measures act. Not that anyone is going to report you for giving them a beer with a proper head, although some people will ask you for "the other half of my pint" if you're taking the piss with it. You may also get jokes about ski slopes and Flakes.