Help with 70% Rule to accurately determine ARV on a property

[Dave Caccia]

New to real estate , Buy and Hold Strategy, own a few properties and have purchased them for cash and made the necessary repairs from my own pocket. Now wanting to move in to financing or getting a mortgage on a new property, I guess my guestion is what is the best way to determine the ARV accurately?

What are some things to take into consiteration?

Is there a web site I can got to research the area where the property is located?

Thanks in advance for your help..