I was thinking of starting with a low bucket size, maybe even 1, for the first render and increasing by 16 for each frame rendered until the time taken to render a frame was longer than the previous frame. Then I would halve value of 16 (to 8!) and subtract that from the bucket size and then use the 'binary chop' technique to hone in on the best bucket size. Once down to 1 I would keep increasing / decreasing by 1 as appropriate as an animation could get more or less complicated as it progresses- although it wouldn't normally 'suddenly' get more complcated, so a 1 fluctuation should be sufficient to keep up with the changes.