I understood it that if I set it to a higher value the chance is higher to encounter the fallen Spire.
No. The 4/10 base setting is how HARD the retaliation from having Fallen Spire on is. It has nothing to do with your chance to encounter the Fallen Spire. Even if you set it to 1/10, you will STILL encounter the exact same chain of events. Just the AI response is much weaker.
Next time, AI, next time!
Fallen Spire is far and away my favorite way to play. I can't recall ever turning the option off since I first turned it on. As you've probably seen, Spire Cities are CRAZY powerful. In fact they are pretty much one of the most powerful static defenses in the game, at least for players. And the best way to combo that is with Military MKIII Command Stations. You will double their damage output, decloak the entire planet, and knockback most things that the city defenses don't outright murder the crap out of. IMO the best way to use the cities are to create two 'types'.
The first type, the one you make with your first city shard, should be 'the front line'. Put it right in the path the AI has to take to get to your other systems. You build one habitation center and the rest reactors. Yes that means your first city will NOT have any shipyards. It also means it will have 6 shields, 12 heavy beam cannons, and 24 lasers. This, IMO, is the best configuration for something meant to stop the AI dead in its tracks. Also, put both the city hub AND your command station right on top of the wormhole leading deeper into your territory. That means the AI MUST remove the city defenses to get deeper into your space.
The second type, which you should build with your second shard, should be one habitation center and 5 shipyards. Put it WELL BEHIND the chokepoint. It's not a fallback point you hold the AI off from, its literally the place you pump out Spire Ship after Spire Ship until you can't build anymore. You don't even need to build modules on the habitation center. Save the metal for more ships. This setup also gives you the most shipyards per city. You can only have as many shipyards in your galaxy as you have reactors. The most you can get building shipyards in every city is 2 or 3. If you get 3 on one you will only be able to get 2 on the next. Thus every two cities, you get 5 shipyards, so it's better, IMO, to just get 5 shipyards all at once every two cities, than compromising your defense by splitting them up over two cities. (Remember, shipyards get ZERO modules, and have no weapons.)
You also get a new 'front line' every two cities. And, assuming you're playing on a map that allows it, you can keep forcing the AI into these chokepoints. Your third shard, which should be another frontline, you put it ahead of the last one, and then you're free to control everything behind it. Even if the AI breaks through your new frontline, they still have to get through the one that you built with the first shard, giving you an in-depth defense as well as massive chokepoints. What's even better, is that the main hub of a Spire City is IMMUNE to damage. Even if the reactors and habs get blown up, the main hub will not get destroyed. You can easily recapture and rebuild your overrun frontlines. On maps like X and Maze, where you can force the AI to take only ONE path to your HW, you gain the best possible chokepoints and in-depth defense in the game. Just make sure to turn of Counter Attack Guardposts, as those will DESTROY your 'backyard' as it were.