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.

In the solar system, for example, [Mercury is in a 3:2 spin-orbit resonance][1] --- so it completes 3 rotations every 2 orbits.  [The spin periods of Earth and Mars, however are almost identical][2] despite different masses and semi-major axes.  Finally, [Uranus has a shorter rotational period than earth --- but is tilted almost 90degrees relative to the orbital plane.][3]


  [1]: https://en.wikipedia.org/wiki/Tidal_locking#Planets
  [2]: https://en.wikipedia.org/wiki/Timekeeping_on_Mars#Time_of_day
  [3]: https://en.wikipedia.org/wiki/Uranus#Orbit_and_rotation