Three Bars (Rule of Thumb)

Adjust power until iwconfig interface reports Link Quality
at the edge of your physical area is ~50/70 (typically "3 bars")

You should adjust access point txpower (/usr/local/bin/hostapdstart)
whenever you move your access point, extend or shrink its physical operating region, or observe interference.

Link Quality
< 30/70
Expect significantly reduced speed and occasional disconnection.
Try doubling power (e.g. increase by factor of 2)
Link Quality
Outlier hosts can tolerate this with minor speed loss.
If hosts in your operating area see this, try increasing power ~50% (e.g. by factor of 1.5)
Link Quality
Hosts at the edge of your operating area should see this for maximum speed and reliability.
Link Quality
Hosts near access point may see this.
If no nearby clients use other access points, there should be no loss of speed.
If this is seen at edge of operating area, try halving power (e.g. decrease by factor of 2)
Signal Level
> -40dBm
Expect dramatically reduced speed and occasional disconnection.
Dont be surprised by an angry knock on your door.
Try decreasing power by factor of 10.