Rotational speeds of planets cannot be calculated/predicted because planet formation seems to be highly chaotic. The spin of planets (both rocky and gas) is determined by many factors, including:
- the angular momentum of the material which was accreted on the planet,
- gravitational interactions with other planets,
- the history of collisions as the planet formed
- tidal interactions with the host star (if the planet is close in) and the gaseous and debris disks while the planet was forming.