Saving up to 42% on a spec build

[Chase A.]

Hey! I recently bought a book called "The Complete Guide to Contracting Your Home" by Kent Lester and Dave Mcguerty (link to book below). At the beginning of the book the authors claim you can save up to 42% when you build a home by acting as your own general contractor and real estate agent. The break down of the 42% is 7% real estate commission, 20% builder markup, 4% savings on material purchases, 2% cost saving construction, and 9% doing work yourself. From what I have seen most spec builders expect 15-20% profit on a build which is much lower than 42%. Does the up to 42% claim sound false or too good to be true? Any responses would be appreciated.

-Chase A.

https://www.amazon.com/Complete-Guide-Contracting-...