Rock,
This is a common question, but the wrong one to ask. The AP can blast a tremendous amount of power and be heard from a looooong distance but the clients will not be able to transmit with sufficient power to reach the AP with a significant signal.
Imagine you're in a stadium listening to a rock band. You'll hear perfectly the music and what the players sing, even if you're in the worst seat in the stadium, but they will not be able to hear a word you say...
To calculate the maximum coverage you need to determine which is the weakest client (in terms of transmission power) you want to support, and the minimum speed you want to support. The lowest the speed you allow, the farthest from the AP you can be. But remember that when one client transmits, the rest have to wait, so if you allow low speeds (like 1 Mbps) your wireless network wil drag.
Want an example of what a slow client can do to your network? check the Zootopia trailer (
https://www.youtube.com/watch?v=iteqM_FeJS8) starting at 0:18... (hint, Hooper= .11ac client, Sloth =.11b client)
