what exactly represents the network load factor that is used for interference calculation? Is there any rule of thumb which value to use?
Ekahau should be the best source of answers to your questions.
If it helps, I can tell you some of the threshold settings I use for an older Fluke/Netscout Aircheck unit. These are user selectable, and based on a red, yellow, green, bad to good scale.
For signal levels they are -83, -75, -64 (dBm). For detected noise floor -82, -88, and -91 dBm. For SNR I use 13, 16, and 25 dB. For 802.11 channel utilization I use 80, 65, 35 percent. And for NON-802.11 channel utilization I use 49, 35, and 15 percent of excessive noise. For retry rates I used 35, 10, and 7 percent ping retries.
These are the numbers I used in my QA lab setup. Other Venues, especially hospitals, may have very different expectations. Especially for non-802.11 noise.
You will often find detected noise floor at -97 dBm or better (lower).
Anyway, hope that helps