There are several simple rules you must follow to determine the extent of coverage you will achieve:

1.    Place access point antenna as close to the clients as possible.  High gain omni antennas work best.

2.    Find location of poorest signal and measure strength and quality.  If it is good you are successful!

3.    To get more signal strength increase gain on access point antenna, every 3dBi doubles power!  Every 6 dBi increases power by four which doubles the distance, since the signal falls of with the square of the distance.

4.    When your access point antenna is maxed out at around 15 dBi (near the FCC legal limit) you can add antennas to the client card or increase client card power.  Same rule, every 3 dBi doubles power.

5.    If your result is not satisfactory you have two choices, boost signal at both ends or increase number of access points.  When using multiple access points make sure to use non overlapping channels.