How to set free fall threshold for ADXL345

Hi I’m using Arduno Uno and ADXL345 to detect free fall. I have come up to here in my coding.

#include <Wire.h>
#include <ADXL345.h>
ADXL345 adxl; //variable adxl is an instance of the ADXL345 library

 void setup(){

          adxl.setFreeFallThreshold(5); //(5 - 9) recommended - 62.5mg per increment
          adxl.setFreeFallDuration(20); //(20 - 70) recommended - 5ms per increment
          //setting all interupts to take place on int pin 1
          adxl.setInterruptMapping( ADXL345_INT_FREE_FALL_BIT,    ADXL345_INT1_PIN );
          adxl.setInterrupt( ADXL345_INT_FREE_FALL_BIT,  1);

void loop(){
          int x,y,z;  
          adxl.readAccel(&x, &y, &z); 
          byte interrupts = adxl.getInterruptSource();
          // freefall
          if(adxl.triggered(interrupts, ADXL345_FREE_FALL)){

When the accelerometer in in a still position my x,y,z readings are as follows 2,-121,206
My question is I don’t know what values to set for free-fall threshold and free-fall duration (even thought I have set sample values here). How do I calculate these values?