How do I verify that my math is correct using BP calculators?

[Emanuel Eyssallenne]

Hello and good evening to everyone!

When using the rental calculator, how do I make sure my numbers are correct?  I use zillow for rental rate, property tax etc. I still feel like my numbers may be off in one area or another.  Also, when estimating rehab costs can you point me in the right direction for getting my estimates more accurate please?

Thank you!