There are several obvious errors that I can see. In order of importance:
1. The floors in WTC-7 did not collapse from the top down as can be seen in any video of the collapse. Indeed, this is something the Truthers constantly point to as indicative of controlled demolition. The collapse began much lower on the building.
2. Nobody has demonstrated conclusively that the collapse actually took 6.5 seconds from the moment the roofline began to fall (obviously we know it took much longer than that when we add in the time from the penthouse collapses).
3. The size of the floors was not 33 inches, resulting in a pile 130 feet high. That's silly, as any picture of the collapsed building will show. Of course, this appears to be an intentional error, as the writer will then claim that the collapse should have taken longer via his computations.