floresta:
Don't get too concerned about the exact value of the series resistor. It's value was determined from the nominal value of the reference voltage which may or may not be 1.24 volts and a desired LED current that we pulled out of a hat.Don
By that I think Don means you can use different R1 values if it comes handy, so long you stay in the reference values for LM338 and the LED.
E.g. using R1=0.5Ohm you get I = 1.24/R1 = 1.24/0.5 = 2.48Ampere which is totally acceptable to your LED (that uses to work nicely even at 2.8Ampere) and to the LM338 that can sustain 5Amps continuosly. Since I don't know if a 5W 0.5Ohm resistor even exists, I'd suggest you use two 5W 1Ohm resistors in parallel in order to get an equivalent R1 of 0.5Ohm.
Anyway, a quick note on you calculations. Pay attention to the LED's datasheet. It states each LED drops 3.6 volts with a forward current of 2800mA (not with 1400mA!). In case you limit the current to 1400mA, each LED will drop 3.3V only.