One can get some idea of whether or not a planet's statistics are within "terraformable range" by looking at what the extremes are for "Earth-like", and assuming that since an Earth-like planet cannot exist with higher or lower numbers, then the planet is not terraformable.
There are always deductable reasons why a "maybe terraformable" planet is not terraformable. In Zieman's table of examples, counting from no 1 at top left:
#1: At 229 K, it's way, way too cold.
#2: Looks fine to me, which is why it's OK.
#3: At 220 K, its way, way too cold.
#4: Um, it already is Earth-like. You can't terraform Earth-likes.
#5: At 3.9993 Earth-masses, I suspect the gravity would be too high. That's from an older version of ED that doesn't actually give surface gravity, so it's hard to be sure. It's got an Earth-like for a moon, so the gravity must be pretty strong. See the Cemiess system for a hand-crafted system with similar properties.
#6: Looks fine to me, which is why it's OK. Strip away the thick ammonia atmosphere and it'll cool down to within tolerance.
#7: Yep, it's a jungle down there, but condense out all that water and you should be within tolerance.
#8: See #6.
#9: 248 K is way too cold. Even removing the SO2 isn't going to warm it up enough.
#10: See #7.
#11: Wow, that one is hot. But comments at #7 still hold.
#12: Whew, even hotter, but still see #7. It's also getting to the point where general atmosphere thinning would be necessary, helping cool the place down even more.
#13: See #4.
#14: See #7 again.
#15: See #6.
#16: See #6.
#17: See #7.
#18: See #7.
The current atmospheric composition is irrelevant, for terraforming purposes, since it is assumed that terraforming tech can strip away or add whatever atmospheric components may be required. The presence or absence of surface water, too, can also be fixed. The orbital period and rotation period (including tidal-locking) are also irrelevant, as is the "quality" of the radiation (so an M-class star is just as capable of supporting life as a G-class, or a black hole for that matter). The presence or absence of moons or co-orbiting planets is irrelevant. I would assume a wildly eccentric orbit would be a disqualifier, but wildly eccentric orbits with an average distance within the Goldilocks zone are rather rare. So in practical terms, the only things that matter are gravity and baseline temperature. The temperature the planet is at now is not a straightforward indicator, as the current atmosphere can be affecting that (eg. CO2, water, methane, ammonia all make temperature go up, SO2 makes it go down, etc).
Looking at the Universal Cartographics Records site for Earth-likes, we see:
Minimum gravity: [0.2266 Earth masses]
Maximum gravity: [3.3944 Earth masses]
Minimum temperature: 260 K
Maximum temperature: 320 K
So, if a planet is in the Goldiliocks Zone (ie it can be given an "Earth-like" atmosphere and have a temperature that falls within that range) and is within those gravity tolerances, then it will be classifiable as "Terraformable". And since stars of the same type have Goldilocks Zones in the same place, it's all about distance from the star.
Now, I have noticed that some of the more "extreme" planets (e.g. very high or very low surface gravity) are considered terraformable when in the middle of the Goldilocks Zone, but non-terraformable if too hot or too cold; A couple of times, I have seen co-orbiting worlds of almost identical surface gravity, where one was terraformable and the other not; the heavier one must have been too heavy for that baseline temperature.
