!!! While the equation works with ~100% accuracy for Syntis planets with a temperature of 0 degrees, I am not certain yet if it works for all races. It has been tested on a couple of nonSyntis planets, and it is fairly accurate, but the temperatures were not as close to 0 degrees as I would have liked. !!!
City Multiplier * (Race Growth Multiplier * Planet Happiness * 792 – (560 * (optimal landmass – planet landmass))) = Growth/h for any colonised planet with a temperature at optimal temperature.

Should be on the dot for planets at optimal temperature. Needs more testing to make sure it is correct, it works for the Syntis race at least though. Also works best for Syntis as happiness is exact, not rounded.

Is able to estimate growth rate for planets around optimal temperature, but becomes less accurate the further it is from optimal temperature
More data is needed to confirm it is accurate (If anybody has nonhome planets with either exactly optimal temperature/landmass, it would be greatly appreciated if you could message me with the following details: Your race, your race’s optimal temperature, your race’s optimal landmass, your race’s growth multiplier, the planet’s happiness, the planet’s landmass, the planet’s temperature, your race’s growth range for temperature, the number of cities and levels of each of them, how much percentage growth your race’s cities contribute to the planet at each level. Sorry about asking for race details too, many details on the wiki are incorrect and I currently only know the stats for the Syntis).
Other notes:
 Syntis happiness is (I’m fairly certain) 99% for every planet.
 Use decimals for landmass. E.g: A planet with landmass 40%, optimal landmass 90% for race (0.900.40). Also for happiness (50% happiness = 0.5).
Example of use of equation:
You have a Syntis planet with 2 level 1 depositors, 1 level 2 depositor, a landmass of 35% and a temperature of 0 degrees.
Syntis optimal landmass is 90%.
Syntis level 1 depositors give a 1.03 multiplier, level 2 depositors give 1.06 multiplier.
(1 + 0.03 + 0.03 + 0.06) * ((0.8 * 0.99 * 792) – (560 * (0.90 – 0.35))) = 357.57568/h
Currently working on getting the full equation, for temperature and not just landmass.