We have decided the LED current will be 300 mA. We choose Rsense with the ADJ pin open circuit which has a nominal value of 1.25 V. Rsense is calculated from the equation R= Vsense divided by the LED current. Vsense is 100 mV ±5%. This gives a value of 330 mΩ or 0.33Ω. The nominal power rating of a 0805 surface mount resistor is 60 mW, so with a power dissipation of 30 mW in this resistor this should be OK. Just check the power de-rating of the resistor over temperature. If the ADJ pin is over driven from an external reference is 2.5 V, then the Rsense resistor must be increased to keep the current to 300 mA. In this case Rsense would become 0.66Ω and the power dissipation in the resistor would now be 60 mW. The reverse is true if you want to set the ADJ pin with 1 V. The resistor needs to be reduced by a factor of 1 divided by 1.25.