i've thought about having it be user defined, which wouldn't be too hard, but i think that deminishes the point of comparing block times a bit.
I like the point that you get your "own" number! - So if I were to decide, I would make it two options for the size of blocks! One where you get your own "perhaps-prime" number! And one where you get the smaller, and thus more efficient blocks (I guess it is quite fine with standard size of these blocks!).