You shouldn't do what every other person is doing (argumentum ad numerum) - especially the FFB feel is very individual.
The force of the countersteer increases with FFB strenght. The FFB force you want to feel and correspondingly the percentage you set in the options are subjective and really your personal choice.
A choice between more counter force and more feel and on the other hand smoothness and easier drivability.
You also have to keep in mind not to fatigue your arms and wrists while test driving 2h or during a 20+ lap race.
Mine is 53% driving the Z4 GT3 on normal tracks and 50% on the Nordschleife. 55% for the 12C GT3 for more feel.
The percentage of FFB also differs with the wheel - I have a Fanatec GT3 RS V2, yours is going to be different.
I for one ignore most of the FFB options in AC - have them at 0%, albeit Filter at 1% to sort out micro stutters from the wheel.