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 --- so it completes 3 rotations every 2 orbits. The spin periods of Earth and Mars, however are almost identical 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.