How does planet quality work?

A class 14 planet raised to 15 through Soil Enhancement will be the same as a Class 15 planet without soil enhancement. Of course, the C15 is 'better' in that you can then build the soil enhancement on that planet and make it a Class 16.
Not sure about the relative value of different numbers.

~SDC~

Planet class determines the base production of a planet, it's max population, and it's base morale. Any planet is viable if you're sufficently prepared, but it will take alot of work with planets below class 15, which is why the tutorial advisaor suggests against it.

The class values are non-linear, for the most part (on a roughly logarithmic scale, if I had to guess).

~SDC~

I posted a file to the downloads section that shows how planet quality increases with modifications. Search 'quality' in the download section

Also, the stickied thread in the "strategies" forum discusses how morale relates to PQ.

It's worth noting that class 14 planets get almost no income, and planets below 14 have maintainance costs (even greater for planets below 10).

~SDC~

Plus people don't like living on uninhabitable planets. This is beacause there isn't enough food or water for a large population and nobody ever gets a breath of fresh air since they have to live in airtight domes.

Great question/s . . .

The improvement % adds planet quality relative to a % of the planets original quality. eg a 22% colinization even will add only 2.64 PQ to a 13 (rounded down I think) it becomes a 15 PQ planet. ALternatively a virgin 26 PQ planet (yes they do appear in the game rarely) collinized with the same even will become a wopping 31 PQ planet.

Interestingly sometimes in GC the program gets the calculations wrong . . . I don't know why it happens all be it rarely.

Some time ago someone said it was a good idea to backfill your teritory when terraforming becomes available. Hence, you can take all the 12s to 14s and make reasonably happy 'tax paying' collonies later in the game without to much bother because the AI won't (much to it's detriment).

This is also a pivotal way of pushing foward your territory and culturaly assimilatiog other AI systems. If you are more technologically advanced and collinize a planet next to an opponents (even if the PQ is substancialy lower) you'll flip the planet quite easily without even having to build a party palace.

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As per the above plus . . .

Colonize planets PQ 14 and above. This will not cost you maintenance. The AI rarely colonizes PQ 14 so save the 14's for last. Initially, do not load up the population of the lower PQ planets because you do not want them to "drag" the economy of Earth or your big production planet. I colonize 14 to 16 PQ planets with 1 colonist per colony ship to avoid overpopulation too soon.

Planet PQ 13 can sometimes be useful as a source of constructors and the occasional combat transport but don't expect tax revenue. It may end up costing you because of need to build multimedia center, virtual reality center, etc. to keep the morale up. I colonize a PQ 13 only when I start my social spending and get the PQ to 14 soon. PQ 13 through 10 (or 11?) pay 5 bc per turn maintenance. I also tend to colonize 13's when they are among a couple of yellows where I might build a production starbase.

Otherwise, I avoid 13 or less unless used in special situations like Cotal/Ginata described or to extend range (the 5 bc per turn is cheaper than a starbase).

Hope this helps. Good luck.