Every card has a minimum Receive Sensitivity to operate at a given speed. This is usually somewhere between -80dBm and -95dBm.
This indicates the amount of signal the card needs to be receiving to operate at said given speed. This value is used by locfinder in several calculations, primarily the Theoretical SOM. As usual, suggestions for future applications of this value are always welcome!
For the sake of consistency, try to enter the Receive Sensitivity your card needs to operate at 1Mbps.
See Freenetworks for a list of some cards receive sensitivities...