As I said, I put my max bucket size at 128.
I don't know if it makes any difference compared to 64, but the renders turn out good in a reasonable amount of time so I've stuck with that.

0.42 is the shading rate because as I move the slider for the min shading rate on the graphic interface, it progresses as 0.00, 0.21 then 0.42 and onwards. So I just chose 0.42 because I wanted higher quality renders and have used that number since.