I wish we would have implemented a scale for reporting aesthetic defects and things associated with PMMA, that way we could relate PMMA injected. I suspect anything about 20 and you will be guaranteed to have cosmetic issue. From what I have seen, anything below 10 and you have very minor gains (we are talking .1, .2). I have though, been extremely impressed with some guys in the 10-20ml range in terms of overall cosmetics in relation to gains.
I think if you really had a clean diet, took plenty of supplements for collagen, and had a 4-6 Inch Cock, a 10 ml injection would give decent enough gains.