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.