(I just ate a Truly Horrible fortune cookie, gah! 8-)
I was working on Vinnies' Next Game, which predictably will be space adventure; as long as I'm not *playing* Dresden Files, then I will design what I want, for which I will then probably not be able to find any players...
I have a system for world-building, which makes sense to me but probably will bore you to tears-
Mass for likely stars are between 1.1 and .8 Sols, with insolation at mass to the 4.5 power, so the insolation would be between 1.5 and .36 sols, roughly; Goldilocks orbits between about 1.2 and .6, respectively, and planet mass is approximately the star mass times the randomly-derived-but-weighted number, the same method which I used to get random star-mass distributions (I did say it would bore you to tears? 8-).
1.1 is 1 / (1+1.1)^3 = ~1/9
.8 is 1 / (1+0.8)^3 = ~1/6
So 3/18 - 2/18 is 1/18, ~5.5% of all stars
Planet mass of .5 to 1.5 E or 1/(1.5)^3 to 1/(2.5)^3, ~.3 to ~.06 or about 1/4 to 1/5, 9/20
1/5 have planets in the Goldilocks Zone
1/18 times 9/40 times 1/5 = 1/2 x 1/40 x 1/5 = 1/400 stars have earth-like worlds, .25%, or about 70 worlds in a 100 lyr cube. Virtually all worlds have some life, but rarely does it equal the complexity of Earth. There are probably a half dozen to a dozen such in the 'sector', and maybe one sentient species per... one in 8 sentient species have civilizations equal to, or greater than, man-kinds' level!
One in 25K stars with sentients, one in about 400K at starfaring or better... my humans have some lebensruam, but lots of competition (one starfaring in every 200 lyr cube)!
The 30 Day Rule of Rule
This is something I've borrowed from GURPS. Size and complexity matter, but speed of communication is king. States more than 30 days across tend to fall to pieces... a decision needs to be made, and somebody will make it, just not the bureaucrat in capitol on behalf of the crown or the republic. The Governor or Mayor or rag-tag leader will make it when and where they need to do so!
I like 1 lyr/day, and there are hundreds of stars in a 30 lyr cube, thousands in a 50 lyr cube, where the interstellar states' reach exceeds its' grasp. That 50 lyr cube even has nearly a dozen habitable worlds...
Speed of travel starts out at 1 C and increases about 25%, or .1 log 10, roughly every ten years. I worked on this last night by lamplight, so the ideas are suspect. This setting uses the Heisenberg Uncertainty Jump Drive, HUJ-D, and it jumps ships off the planet. Reaction drives powered by shipstones and using water for reaction mass allow ships to land safely, under power. HUJ gets them off-world and between the stars, at 1MWhr/tonne/lyr. Early mass space travel based on cheap, reliable access to space (CRATS), leads to the development of the Solar System, Cowboy Bebop-style.
In the second century FTL, Sooners start to settle stars and worlds w/in 20 lyr and develop in complete freedom to fail, or be swamped by later colonists. Most early colonies failed and are outliers or mildly inhospitable, easily terraformable planets. Many later colonies fail as well, but there are more of them, learning from the early mistakes (so that they can make some bright, shiny new ones!).
Who are the Sooners?
Who are the Johnny-Come-Lately's?
What is happening, back in the Solar System? (Solar Commonwealth)
I'm figuring on the early 3rd century FTL to be the point at which things shake out; interstellar commerce begins to make sense, more colonies are founded, and the Human Interstellar State decides how all this is going down, first in the Core 20, a 20 lyr cube, then in the twenty-six additional sectors formed adjacent, in the Core 60, with a baker's dozen habitable, good worlds, and a few dozen not so perfect worlds...
The Habitable Worlds Inventory
I have a suitable gazetteer of local stars, and I will use it, but first I will randomly create about two hundred worlds and place them randomly, then rationalize their actual placement according to real star data...
Habitable worlds, and not so habitable worlds, are of varying usefulness, based on arable land for living and agriculture, temperature, etc. I have ideas for sectors and ideas for colonies on habitable worlds, all influenced by Mark Steins' excellent 'How the States Got Their Shapes'.