Fanatec recommended settings are fine for me @ 75 gain ingame (8Nm). Except for the McLaren 720 as this car has, afaik in reality, a very strong powersteering, which is mirrored in ACC accordingly . I use 80 gain in this case. Not too much as a gain too high takesxaway a lot of the subtle signals from the tyres. Less is more in this case. I am trying to drive a GT racecar at it's limit, not to do power workouts in the gym.
Steering angle 1080 vs 900 makes practically no difference as ACC chooses the correct angle for each of the car models by itself (Aris) and , other then in AC, there are no older cars in ACC for which you would need either of the 2 wide angles. However, the only requirement is that angles in the fanatec wheeldriver and ingame have to be identical. If you drive only one car day in and out, you could probably set it's correct angle in both, the driver and ingame.
BTW : For all other sims ( 5 ) I use the "Auto" setting in the driver and all the Fanalab/ Fanatec Forum recommended settings. In most other sims ( even AC ) you can monitor and adjust the FFB setting / gain by car even during driving, which is a way more elegant solution then in ACC, where you have to save the complete controller setup for each car. Unless you drive only one during a season...