Calculating expected receive power
If you want to estimate how much power you will receive you can use the Friis transmission equation to get a rough estimate. 

$$
P_r = P_t + G_t + G_r + 20log_{10}({\lambda \over {4\pi R}})
$$

The \$P_r\$ is the power received the \$P_t\$ is the power transmitted that you determined you're sending, the \$G\$s are gains of the transmitting and receiving antennas respectively (in dB) and the final bit is the isotropic antenna equation, which will tell you how much power was 'lost' because it was not directed at your receiving antenna.