| Mynex |
If you want to calculate the orbital rotation (how long it takes the planet to orbit it's sun), you're looking at some serious trig... If you want the planetary rotation, that you'll have to make up.
There is no 'simple real world formula' out there... I've been putting together things to come up with density, diameter, year, gravity, star spectral type, etc into excel... and I've learned more about astrophysics and remember more trigonometry than I probably ever wanted to know (well, before now, it IS fascinating as hell!).
There are some RPG supplements on DriveThruRPG/RPGNow that gives you some 'roll these dice to get this'.. from galazies down to moons... but they're overly complicated for RPG casual use (note, I said casual, if you like all the stats/numbers, they're definitely up for that!).
So.. yea, the donjon one works well enough for casual use/info, but you'll still have to make some things up.