Even with perfectly accurate data on the project and excellent reasoning, outside factors like viruses and changing market predictions can throw a wrench in things.

@Hattie Dizmond  different parts of Irving fall under 3 different school districts - Irving ISD, Coppell ISD and Carrollton-Farmers Branch ISD so a blanket statement "generally poor quality of the schools, I would not purchase in Irving" is far from accurate in my opinion... not trying to pick a fight just saying :)  Coppell ISD annually ranks amongst the highest. 

Essentially you need to understand the ROI associated with real estate (and Ideally investing in general) and how cap rates, equity, appreciation, and liquidity all play different roles.   ..and when you advertise your properties be sure to quote the most accurate numbers you can.  

Also, really important you know accurant rental rates in the area as you don't want to get caught with revenue much less than planned.

I've been through a number of real estate transactions in NJ and @Jonathan Greene's depiction is accurate.

There is just too many moving pieces to accurately predict the outcome.

I've been doing my estimates in a spreadsheet, but it takes a long time and more to the point I'm still not very consistent or accurate on how many man hours different things take.

Again, wether that is 100% accurate or not I’m not sure.

How do you go about finding the most accurate data? 

In your experience, BiggerPockets Community, how accurate have your experiences been with the GP's projections on returns.