Think fodandahalf has just about nailed it for you already.
While trying to figure out what hairline I should have myself, I looked into facial proportions, and it seems the general rule for the lowest hairline, is if you divide your face into equal thirds, so that the lower third is the bottom of your chin to the bottom of your nose, and the middle third is the bottom of your nose, to the bridge of your nose where your eyebrows are, then your hairline should be from the bridge of your nose, to wherever the top of the 'third' happens to be.
If you want to go a bit higher than that, you can draw the hairline anywhere between the top third mark + 1cm. Just depends on how youthful a hairline you want. Right at the top third line gives you a 20something hairline, heading to the top third line + 1cm, gives you something closer to a mid-thirty something, I think.