This is interesting to read because when I did hairline design with him, I think it was too juvenile for my preference. He seems to follow the 1 finger above the frontalis muscle rule, but I felt that for my head and face shape, I thought that it would be too juvenile.
I think if you're going below the 1 finger above the frontalis muscles, it would maybe be pretty juvenile, but the 1 finger can vary. I think it depends on the face and head shape, for me I felt going well above that would be ideal for my head and face shape. I think for a guy like Paul Rudd, who has a big forehead but no hair recession I could notice, for sometime like that going near the bottom of the 1 finger would be ideal.