Assuming full 14 activity points every two weeks, there are only 8 points to be gained in the 19th period (as evidenced by your activity score of 1022) so I think that means only 8/(1030-775+1) = 3.125% of wannabe-Legendaries can be sure.
Now you're assuming theymos made a linear scale for the chance of reaching Legendary between 775-1030 Activity.
From what I've seen, I expect something like this:
b*(1-0.15*s/b)