Answer by Thomas Zerbarini:
Tom's got a great answer.
I'm also a very big fan of pulling out the spaghetti charts (performance charts) to confirm my aircrafts capabilities. It's just good piloting to master your aircraft.
Here are some examples of the charts pilots, flight engineers and dispatchers use to calculate performance numbers. We used to do these calculations ourselves before standardized performance manuals and computers. Despite the help from computers, good pilots still keep tinker with the old performance charts to stay proficient in using these charts.
You can google more here to see them better: