I've read dosing guidelines based upon weight...and I guess it's a starting point. However, many things affect the absorption of the meds and you'll find that some people require much larger doses than their weight would indicate.
Better to dose with the thought of complete symptom elimination....which will most likely result in thyroid hormone levels towards the top end of the range.
My doctor started me out on the standard 25mcg dose....and it actually brought my thyroid hormone levels to the top of the range (admittedly, I had debilitating hypo symptoms with levels just below mid-range).
That dose "lasted" two months....and, almost one year into taking thyroid hormone replacement meds, I'm still not sure if I'm at my ultimate dose - all I care about is feeling good.
Thankfully, I've been able to achieve that since I found my optimal thyroid hormone levels and my doctor works with me, adjusting my dose to keep my levels where they need to be.