Terraforming could be an automatic function of the Construction meter. Each turn, the enviroment of the planet moves X degrees towards the inhabitant's preferred enviroment; the number of degrees computed based on the Construction meter.
I'm not too keen on using the construction meter for this purpose. IMO intentional terraforming should be done with specific terraforming projects. (There could be some unintended terraforming, ala global warming, but for any race / envirnoment that's overpopulated or over developed).
Classfication (Optimal, Poor, etc. etc.) would be maintained for human digestion, but the actual numbers for EP's effect would be generated by a function based on distance from enviroment to preferred enviroment.
I personally like the idea of making the "real" environment classification of a planet much less granular than the spokes on the EP wheel, but I figured this was quite against the purposes of the wheel as it was intended to function...
The end result: any race will slowly and automatically change a planet to a type more suitable for that race.
Again, terraforming shouldn't be automatic in your favour.
The moo2-esque terraforming project would change the planet's base type. Then, the planet's actual enviroment would slowly move towards it's new base type.
IMO it'd be better to have terraforming projects cause the change in environment directly. Depending on your level of advancement in whatever relevant environment / terraforming techs there are, the rate at which you can change a planet would vary.
The "official" environment classifiaction of a planet would depend where its actual environment "angle" is relative to the nearest "pure" environments (the current classes like Toxic, Terran, Barren). Assuming even spacing between environments around the wheel, anything within 1/3 of the distance between environments on either side of a pure environment would be classified as that environment. The stuff in the middle 1/3 would be labelled as transitional environments... like "Toxic-Swamp Transitional" for the player.
IMO, planets should also remember their original / natural environment, and the maximum distance away from the original environment you could terraform with a given technology would be a hard limit. As well, the rate of change of angle/environment would fall off as you moved a planet further from its natural environment. Further, terraforming would need to be maintained, and the cost of doing this would increase with distance from the natural environment, and the rate of decay back towards natural would increase as you move a planet further from its natural state.