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There are many factors that can come into the equation when aiming for a certain length. Some of them include: how the length looks straight or natural, how practical it is to maintain, how it will fit your face, whether you want a blunt cut or are content with a V-shape, layering, etc.
You can have two bra-strap length girls with one having longer hair than the other because of neck length, height, the length of the back, etc. So a hair goal can be relative to perception, features, body and aesthetic preference.
For me, waist length hair was never a desire. Even as a child I thought Rapunzel was over-doing it. I've always preferred hair that hits mid-chest length in the front and somewhere between Bra-strap and Mid-back in the back with a blunt cut. That is my ideal length-straightened. I think it would be a lot of fun to have hair down to the waist, but I don't think it would fit me and I don't think I'd enjoy the detangling process.
What look are you trying to achieve as far as length?