Will you be missing something? yeah you will, it's called SQ.
A good tune takes an experienced tuner at least 2-3 hours. A quick tune takes 1 hour. A 15 minute tune? just nah
The biggest flaw in this whole video is using a single, static mic. It's going to capture a shed load of comb filtering effects that you just won't hear with your actual ears, and that's going to seriously compromise the response of the system.
Measuring a single point in a car is just a waste of time. You MUST use the moving mic method. I honestly have no idea why anyone is still using this mic technique from the 90's.
That said, go for it - you have to start somewhere, and it will give you a baseline to improve upon.