Nah I think 20ml is good. I got nothing when I first started pumping. Even now I normally only get 30ml after a feed or nothing if he's drink it all.
Have you seen a lactation consultant? They really helped me with recognising when he's drinking and when he's not and what to do if he stops sucking. I went to the counsel one and she was fantastic. I didn't go at first cos I thot I didn't have a problem with latch on but realise it's much more than just latch on technique.