Last time, I talked about working out every culture's homeworld, how advanced they are, and when they achieved FTL tech. So, knowing that, the rest is simple, right? Whoever has the most advanced tech, who gets there first, they're the most powerful, right?
First of all, there's a matter of location. Where species are in terms of not only each other, but other worlds, and what those resources mean. A species that hits the stars and finds they have thirty-some odd other stars within 10 light-years, and most of those have vibrant solar systems chock full of potential resources-- they're going to have a different interstellar experience than a species whose closest star is 15 light-years away.
Even with that, I make deliberate decisions, since I'm the worldbuilder here, for example, if a species takes an aggressive or isolationist stance in exploration. If they focus on staying close to home, or perhaps try and spread themselves thinner than they should.
For example-- I've set up my FTL rules so increases in velocity jump in increments. Ships form a field around themselves, and the level of that field determines their velocity. An Alpha Field is the slowest FTL field, then Beta Field, Gamma and so on.* This gave me an easy way to benchmark future tech-- I can measure
general advancement of cultures compared to each other without having to
get too specific about what that means, techwise. A culture that can
form a Zeta field is more advanced in general than a culture that one
can only form a Delta field. I don't need to figure out the specifics
of hull composition or missile yields for every different culture. So, every culture is rated by their maximum speed. And with that, I can estimate a reasonable radius a species can maintain control over. That Zeta culture can spread out, say, 30 light-years from home, while the Delta really can only manage 16. But at the same time, a culture's nature might be to push themselves. They might have to push themselves to reach resources they need.
So, that process of building every culture out from their homeworlds, figuring out what they build and where, who they bump into, what they decide to do when they bump into each other-- that's the real gearwork of the worldbuild here. It's not sexy, and it's not stuff that really appears in the text. Iceberg rules apply in spades here.
Then comes the next big step: figuring out who the Big Dogs are. Of course, the Big Dogs are the ones I chose-- especially the ones that are big dogs because they joined up to form larger empires. I've got eight First Tier* level powers, and four of those are joint-species collectives of some sort or another. And also, that isn't just about tech level, but about dominion and influence. The Colmerohn are more advanced than the Zutheka-- but the Colmerohn are slow and deliberate, and would rather withdraw than engage in conflict, while the Zutheka are hyper-aggressive conquerors. So the Colmerohn are a Third Tier power, and the Zutheka are First Tier.
The next step: figuring out the "borders".
*- I've worked out the math, but you knew I did. An Alpha field will make the trip to Alpha Centauri in 2.95 years. With a Delta field, it takes 40 days. An Epsilon field cuts that down to under two weeks. If you managed to form a Xi field**, that trip is slightly over a minute.
**- No one has a Xi-field level of tech.
***- There are also three Zeroth-level, i.e. very high tech isolationists.