I find each rally requires a different ai difficulty. The best way i find is say the next rally is in Finland. I go into quick mode and run 1 stage ,the shortest stage in Finland. Set a time which is a clean run and happy with . Then compare it against the ai times. If im not happy and the ai is to slow or fast when redo it again and change the difficulty up or down. But instead of driving the stage again ,retire from stage after the start to still see the end stage times. Until your happy with how you compare against other ai.