It should read " not that well." - far normally refers only to distance. To say somethings is far is to say it is a relatively large distance away from you. Figuratively far would mean when someone has crossed a line or a boundary. In the example above A: can you understand movies in English? B: I wouldn't go that far. The response "I wouldn't go that far," means that B cannot understand movies in Engish very well so he would not go as far as to say that he can. Hope this helps!

B