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.

You're rehab budget will NOT be accurate... 

I guess it I s find any cash flow properties with it;( Thats not really accurate if you are buying multi unit.

You can really get a bid from a contractor at any phase; however, the more information and detail you can provide, the more accurate your quote will be.

I suspect virtually all my bullets are accurate to Portland.We were also under contract for a high end ADU value add (Cost $1.05M for ARV of $1.45M) in March but there was a challenge to over come on this particular ADU addition that we pulled out. 

If I were a better coder, I'd grab rental listings and make a neural net to evaluate rental comps for any given property to get a more accurate picture than zillow.