802.11 RSSI value displayed from ifconfig(8)

[Eduardo Meyer]

Hello, I would like to convert RSSI to dBm (in fact I would love if
ifconfig cound display SNR in dBM), so I would like to know if this
formula is any right:

/* Calculate the RSSI Value */
V_RSSI = 3.0 * (RSSI_VAL/1024);
RSSI_dBM = (int8_t) ((-50.0 * V_RSSI) - 45.5);

And if there are any other tools which can help me finding out signal
stenght, when I am acting as an access point.