Congratulations! If you have followed the instructions in the preceding articles closely then you are likely on the homestretch of your quadcopter design!
We would now like to mention a great source for back-checking your work. It is the multicopter edition of Ecalc which you can find here.
Now we know what you might be thinking... "Why didn't I just start with Ecalc!?"
Remember the difference between a hobby design and an engineered design! Anyone can waste hours on Ecalc plugging in every combination of variables possible to find a setup with the longest flight time. But after all the work, that person may at best end up with a quadcopter with little to no chance of being able to optimze it or build a better one in the future. And that person at worst may end up with an imaginary multicopter literally impossible to construct. You do after all have to fill in a value for weight/mass that is realistic!
Also keep in mind that people are flawed and there is no substitute for diligence. We mentioned elsewhere on this site that you should not re-invent the wheel. But we would like to balance that thought here by saying it's terrible for the blind to lead the blind. Ecalc is a great tool, but do not consider it to be absolutely without any errors. Daniel has already seen one forum post where somebody got seemingly faulty results from Ecalc. So please use Ecalc as a safety net and not a crutch.
That all being said... Once you have some options for your quadcopter, you may enjoy plugging them in to Ecalc to see the different results they may obtain.
Ecalc even has some useful safety features that may save your design in the case of an oversight.
We encourage you to circle back through your design like we mentioned at the beginning. Make small changes and observe how those changes affect the rest of the design. Your change in motor selection may result in better efficiency at a given operating point, but the motor's weight may significantly change the operating point and actually land its efficiency at a lower number. You need to follow through on all the implications!