I wrote a simple script to get the current RSSI from my Macbook Pro and using the Mist API to get the RSSI associated with my MAC address and run continuously. You will need to ^c to break out of the script. Keep in mind the Mist AP reports its stats every 60 seconds or so.
Inspiration from https://github.com/mike-albano/rssi_getter
This worked with MacOS Mojave 10.14.3 on both a Macbook Pro and the new Air.
This is the script.