- Took the rent rates directly form redfin.com link and monthly insurance $75 (rounded up from $62)- For expenses water/sewer ($100), garbage ($50) these are arbitrary numbers I selected without much research (not the best method for accuracy i know :/ ).

This is such a niche thing that I doubt many of us here can put a value on it with 100% accuracy. 

The hardest part is actually just finding the absentee-vacant homeowners address and then verifying it's accuracy, after that, any honest, trustworthy and straight forward handwritten letter will do.

Don't go  with Zillow estimates they vary in accuracy for these kinds of properties. look for something close in features that sold.    

Make sure to get the current rent contracts and also in the sale contract itself make sure you have a clause guaranteeing the accuracy of the rental contracts.

Here's our go-to list:ZillowHotpadsCraigslistRentometerWe have found that each site has a little different information and accuracy depending on your market or neighborhood.

You'll at least get better accuracy there.

You can gather more data but than your just getting a poll of polls and you haven't necessarily increased the accuracy of your answer 

I have a front row seat to some of the things I've said in life, and trust me, there was minimal accuracy in some of it.  

I do not have time to calculate with perfect accuracy but using rule of 72 I show an 9+% annual return on the appreciation versus purchase price.