Modern 2.4g receivers have Link Quality instead of RSSI. It doesn't do you any good to see a high RSSI when flying a long way away, just because you are flying over someone's wifi. You would need to know how high the RSSI of your packets is when compared to background noise.
You use Link Quality as if it were RSSI except that it will be close to 100% most of the time. When it is below 100% most of the time you should turn around, because you are probably at 90% of maximum distance. You can test to see if it changes by turning off the transmitter and it will drop to 0% if configured correctly.