I used to live in a small town right after Capannuccia, and went to work frequently with the public transport to avoid parking in the city. Though I never used it later than 8pm, I do know that it runs later because I have seen the buses even around 11:30pm.
Personally, coming out of that area (Capannuccia) I would always pick-up the bus in Grassina (number 32).It is about a 40 min ride into the city center, though I would get off around Piazza Beccaria and walk into towards the center. There is free parking behind the square (though the lot does close around 10ish) or in front of the main church. It runs frequently and has late runs in the evening.
You can take the same bus or even bus number 31 if you park closer to the city, which will give you more flexibiity with the times. For example, I have left my car in the San Marcellino sports center along the route (very convenient - the bus stop coming and going is right across the street) and many times I have left the car in the church parking lot just a bit further up and only 50 meters from a stop.