Don't worry about what it was listed for previously, as long as it works within your calculations. 

BiggerPockets also has a calculator to analyze deals, and I highly recommend you start this as soon as possible, even if you are not ready to buy.

The biggest advantage of using someone local to where you're buying is that they will understand taxes, insurance, and similar items very well when it comes to DTI/DSCR calculations and payment quotes, and they will likely have a local network of service providers for ancillary services (survey, inspections, etc).

Quote from @Bonnie Griffin Kaake: Here is a simplified calculation that some may be interested in: ADU cost $200K, annual rental $21,600 ($1,800 per month) = almost 11% per year.

Have you seen Scott Trench's sell or rent calculator?

You need to calculate your number.  

At the water heater the cold line in will calculate all usage.

Your IRA custodian should have advised you by now and calculated the amount for you.

I felt he was still calculating, examining me in this strange way that sends shivers down my spine when I think about it. 

With BiggerPockets Pro you can access a lot of great calculators & tools for doing exactly what you mentioned above.