Wireless Sensor Network Fundamental?
This might be what you are looking for.

http://en.wikipedia.org/wiki/Friis_transmission_equation

You can solve for Pr for the receive power or Pt for the transmit power and R is the distance.

This equation has antenna gain and frequency factored in so I'm not sure if there is a more simplified version with just power and distance.

Hope this helps you get started at least!