To calculate your link you will need the following:

• RSSI value of your wireless bridges.
  
  
• Distance between links.
  
  
• Power mW or dBm to be output by bridge.
  
  
• dBi of antenna's to be used.
  
  
• dB loss of cables, connectors, lighting arrestors etc.
  
  
• Frequency to be used (2.4gHz or 5.8gHz).
  
  
• FSPL (Free Space Path Loss).
  
  

Free Space Path Loss (dB) = 20*Log10 (MHz) + 20*Log10 (Distance in meters) + 32.4

Once you have your calculation you can add your SOM (System Operating Margin) and see if the RSSI is enough to sustain the data rate needed for the link!