Are levothyroxine dose quantities unreliable?

No. As long as you stay consistent with your medication. That is to say, the brand synthroid may not be bioequivacle to the generic levothyroxine, so it is not recommended to take one one month, than switch to the other, etc. Stick with one specific medication, take is regularly as prescribed approximately the same time each day and get your TSH levels followed as per your doctor and you should be ok.