The aero data comes from Cars Guns and Digits youtube channel. I have actually gone back to using the Forza6 tuning app because the base tunes it provides with max stiffness and max understeer combined are very good. If you use the app be sure to set your aero with the above mentioned method because the app asks for the downforce values to determine proper set up. Very easy to hold the line and get the nose pointed in the right direction at high speeds using the app tune. Some say using the app is sacrilege however since I picked up the app I can basically guarantee myself a top 3 spot in pretty much any race with little trouble. Once I got away from the app I would be lucky to finish 6th so I had to revisit it. Besides, even in real life they are using computer programs to dial in their race cars to peak performance so I thougtht hey why cant i ? Lol
If you dont have the app just make sure your taking into account the weight of the car and weight distribution. Visit cars guns and digits youtube channel. His methods will allow you to create a fantastic base tune and you dont have to be good at math to understand.