Doing the math that is available it seems like the rate to catch something like koraidon in a beast ball is around 0.16% and 1.26%, and this is without actually understanding how the equation actually fully works.
I find this concerning because when beast balls came out, it came out that the beast ball beldums people were trading were originally hacked, because the capture rate algorithm made it so that you had an effective capture rate of 0% regardless of hp or status. This is the case for many heavier pokemon.
I have a strong suspicion that BB Legendaries are too low chance rate to trust.

at full HP. Doubt that's the most efficient but it was pretty mindless.

