RSSI (receive signal strength indicator) is sometimes on a scale of0-255, 0-100, or 0-60 depending on the chipset. The original conversion method is in the 802.11-1999 document (I'm too lazy to find it). All the manufacturers have their own idea of how the conversion should be done. See:
"The IEEE 802.11 standard defines a mechanism by which RF energy is to be measured by the circuitry on a wireless NIC. This numeric value is an integer with an allowable range of 0-255 (a 1-byte value) called the Receive Signal Strength Indicator (RSSI). No vendors have chosen to actually measure 256 different signal levels, and so each vendor?s802.11 NIC will have a specific maximum RSSI value (RSSI_Max). For example, Cisco chooses to measure 101 separate values for RF energy, and their RSSI_Max is 100. Symbol uses an RSSI_Max value of 31. The Atheros chipset uses an RSSI_Max value of 60. Therefore, it can be seen that the RF energy level reported by a particular vendor?s NIC will range between 0 and RSSI_Max."