It is based on your blood test results. TSH, FreeT4, FreeT3.
Usually the medication is started at a low dosage and stepped up every other week or so untill you have reached your target dosage based on your test results. Then re-testing is done after a month or two to evaluate if the dosage needs to be altered according to the range of your test results. Thats usually how the initial dosage is given. To get a proper amount and combination of medicine to maintain satisfactory blood test ranges and physical/mental wellness can sometimes be more complicated to dial in.
As well as how you feel and lab values... I also know there is a formula...However I dont remember what the formula is But I know that it is a formula involving your weight and it is to show around what dosage you should be at for your body weight. My endo told me about it but I cant remember... go figure