I think Peat has made clear that Vitamin A is biphasic (too little is anti-thyroid, as is too much) and the amount you need depends on how many calories you are metabolizing in a day. No study is likely to be designed with this in mind.
For example, Peat recalls that when he was metabolizing 6,000 calories in a day he needed I think 50,000 IU of Vitamin A, but when he became more sedentary and metabolized 3,000 calories in a day, the amount he needed dropped to I think about 10,000 IU.
Since it seems most people here and the forum have relatively low metabolic rates, and can metabolize only 1,500 to 2,000 calories a day without gaining fat, there will likely be a smaller therapeutic window. For this reason, you should experiment cautiously, and try to increase your metabolic rate.