I know they are starting to loosen up a lot of the restrictions as this building methodology.

The reason being that the houses were older than I'm accustomed to and they use different building methodologies up there, so I wasn't able to assess things like the well, foundations on clay soil, rubber roofs, etc.FWIW, I was over budget by about $25K on the first Milwaukee rehab -- about $10K of that was stuff a really good inspector probably could have caught (structural/framing issues), but the inspector missed it and so did I.

Anyone know how this became a standard - the methodology behind 2%?

From a methodology perspective, I'm using the relevant fields of 362,386 county property card records in a series of queries to obtain the results.

Then, as the deals flow in, dissect each one using your preferred analysis methodology.

As the book recommends, you'll want to find out specific prices for your area, the types of contractors you'll be dealing with and the types of houses you'll be working with -- the book is just a methodology to do that with general ranges of prices. 

The Zillow Research data is from June 2019, as Zillow changed their methodologies after that, causing a discontinuity in the data.

Calculating your income requires proper methodology, knowledge of guidelines, and often a worksheet to help with the calculations. 

Their philosophy was that they have made and continue to make millions and millions of dollars, so they want all agents to stick to their methodology for generating new business. 

@Michael S Dawson While it may seem like a simple concept, it's actually a very complex process that requires significant documentation as well as specific methodologies.