Brooks Brothers offers a great guide to tying necktie knots. Great except in one regard: for every knot, no matter the size, the reader is instructed to ensure that the rear blade (the narrower of the two strands of necktie hanging from the knot) is the same length as the front blade--the one that everybody sees.
This is categorically bad advice. The only time you should worry about the rear blade is when you make sure that it is not longer than the visible front blade.
I honestly have no idea why they say "blades should be the same length on every single knot". If you follow that advice with a Full Windsor, you will look like this rather than this. Occasionally, the blades will be the same length when I wear a four-in-hand, as they did for me the other day
but that is neither here nor there. Men's chests are different lengths, whereas most neckties have very similar lengths. Just worry about whether the front blade reaches your belt, and you'll be fine.