Data is similar to statistics, you can prove/disprove whatever you want with a little manipulation. Seeing as everything in flying is what you think the aircraft can do then add 30% for safety, it all makes calculations to 3 places after the decimal rather academic doesnt it? In reality you will get a given speed for a given power setting and usually it will be somewhere near what you expected. If it is way out then you do an extra FREDA t's and P's check etc to make sure you havnt left the flaps or wheels down or the prop in fine.
And to be honest is anyone going to re-calc for the actual temp and pressure of the given day, assuming you have calibrated instruments able to measure the parameters to the required accuracy? So as i said, take the book figure, add 30% and go flying
