How to calculate RSSI across distance in free space mode?

Here you may go for RSSI calculation

RSSI stands for Received Signal Strength Indicator. It is an estimated measure of power level that an RF client device is receiving from an access point or router. At larger distances, the signal gets weaker and the wireless data rates get slower, leading to a lower overall data throughput.

The RSSI is measured in dBm and is a negative value. The closer to 0 the better the signal is.

• Typical LoRa RSSI values are:

RSSI minimum = -120 dBm.

If RSSI=-30dBm: signal is strong.

If RSSI=-120dBm: signal is weak.