Real world performance to horsepower figure! The app only needs your slope corrected 100-200 km/h time and the total weight of your car (passengers included), to give you an accurate result the app needs precise weight and time data.
The app is more accurate with automatic transmission cars.
If you drive manual and to get a 100-200 time you have to shift more than once or your shift is slow, the app will give you a pessimistic results.
The app calculates horsepower from a database of more than 300 cars so it consider a pretty good average of cx, rolling resistance and drivetrain losses.
Please bear in mind the app gives you an estimate, do not expect 100% accurate result, but within 2-3% margin of error if data inserted is real, while entering approximate data will give you a wrong result.

Feel free to contact me to report bugs or any other issue.
Enjoy!