My question is, how do I determine that with a reasonable degree of accuracy? 

In regards to plugging numbers into a calculator and not coming up with the same number, without reference to what those numbers are and how you tried to calculate them, nobody can speak to the accuracy of your number crunching or the post which are referring to.Perhaps make up and example and some folks can comment to help you understand.

Of course props that haven't been rehabbed yet will look bad, but you should be able to get a good idea of the quality of the area, which will also indicate the accuracy/honesty of the people pitching the property to you.Ask a billion questions.

It becomes practically useless as far as actual accuracy after the first year or owner ship do to the complexity of all the new variables involved.It falls into the 50% rule/guideline category, rough but of value in certain situations and under some circumstances.

Zillow has a lot of it, but the accuracy is shaky. 

NO the Zestimate has a 80% accuracy when accounting for a 20% variation. 

If I were the one deciding on which deals to pursue, I'd spend the majority of my time validating the accuracy of the assumptions sitting in that sheet.Also, you should really password protect access to your VBA module if you don't want people snooping around your hidden sheets/cells.  

@Patricia Velez it is almost impossible to answer your question with accuracy. 

But what did you use when you were first starting with limited funds, and what was the accuracy like?

Sort term, id find a realtor for the best accuracy.