You are the one who is completely wrong.
The dosages most certainly do start out with a "best guess" in the pre-clinical phase. For any given drug we would guess at the doses based on our calculations, and give a range on either end. If we thought the effective dose would be 1 mg/kg, we would run an LD50 of doses much higher than that, 10mg/kg, 100mg/kg. Sometimes our guesses would be wrong, and the low dose animals would die as a result. After figuring out a safe dose, we would then split it further into different dosing groups to test the effectiveness of the compound. Most people in the field are underpaid, overworked, and do sloppy work
Please talk to someone who has actually worked in the Pharma research field, they will tell you most of what they do is just guessing.. why do you think only 1 in 10,000 make it through?
It's all a guess until the end, and even then we're often not sure. Look at anti-depressants. We still don't know how a lot of those work but prescribe them to millions of people regardless. How do you think they determined the dose when they don't know how the drug works?