legendary is somewhere 100%, had a guy with lvl 23 ft in few seazons, after he got 19+ fts, never missed single ft:) personally i think lvl 9-12 of fts is excellent, guy with 10 fts shoots around 77%
I have a guy that went 1-2 in a scrimmage a couple of games ago that has at minimum 21 ft (he was legendary already when I acquired him and he has popped once in FT while on my club). He's also had games where he's missed 2 FTs before the pop to 21 (a 4-6 and 6-8). So while the percentage at legendary is usually quite good, it's still some ways from 100%.