It mostly depends on how robust your machine is. Offline AI Traffic imposes a load on the CPU just as Online Traffic would (without the added complexity of Internet connectivity and quality).
When I operated with Offline AI (this was when Live Traffic was not working well), I would go to about Slider 30. That gave me plenty of traffic within the 50 nm radius. But I’m not running a beefy machine.
when it comes to AI, there is not the one perfect setting. Performance is depending on so much stuff that it is hard to tell what will be the best setting for you.
I suggest to test it on your system and check for yourself. Start with around 40-50% and then go up/down depending on your FPS.
For this test I would chose a medium size airport, to get an avarage result. Traffic at CDG will have far more impact on FPS then compared to regional airports in Norway or so