No one can estimate the FMV of a house that closely.
The property tax assessor in my county estimates the FMV of EVERY house in the county that closely.
(And has to defend their estimate to owners who challenge the assessment.)

I think your point really was that it's possible for two different people to come up with two different estimates that are both valid. IMO the best thing you can do is get a professional (ie. RE agent or appraiser) opinion. Then if the IRS doesn't like what you use for a number, you can point to the documentation from the expert. (And if it's reasonable, the IRS probably won't bother beyond that point)