Should you normalize RGB values by 255 or 256?

It looks dirty, but yeah, it's 255. If this is unintuitive you can force the intuition by looking at the degenerate 2-bit case where the only integer values are 0, 1, 2, and 3, and then brute force the corresponding int->float conversion values. If you don't want extremely obviously wonky behavior like obviously-uneven, or black or white not being black or white, you end up with 0.0, 0.33..., 0.66..., and 1.0. And then the inversion of that involves multiplying by 3, not 4 (2^2).