The RSSI just gives you a number for the signal strength. The signal strength depends on more parameters than just the distance. Obstacles like walls, windows etc., reflections, the location of the receiver/transmitter, if it's in a pocket, close to the skin, lying on a table, antenna design etc.. All in all this is just a matter of testing. Try and see if the RSSI values in an environment similar to where it's supposed to operate give you sufficient info to calculate the distance.
i keep my advertisement interval as 100ms and power transmission level as 0 DB. just consider that there no obstacles. i want to measure the how far my device from central with RSSI value
