And if you really need to get down to the hundredth of a degree you probably should be using jacketed bullets. There is no way you will be able to mix and maintain that kind degree of accuracy without a full lab setting. I am not sure a foundry could do that with lead. Maybe steel or aluminum but if it is possible with lead it certainly is not feasible.
Thinking about something, when you are mixing all your ingredients are you fluxing? I am thinking from the sounds of what you are saying this is not pristine lead you are working with so you need to clean it up a bit by fluxing so you are probably losing 1-2% here and there in the dross from fluxing.