How do resistors limit current for an LED?
Since the forward voltage of the diode varies slightly according to the current through it, more accurate value of the current can be found out by - 


Plot forward V-I chara of the LED (from its datasheet).
Plot the load line of the series Resistor for the given value of R.
Find the meeting point of the two graphs. It should be the operating point of the LED + series resistor circuit. 



Vs - Power supply voltage.

V,I - Forward voltage and current through the LED.

Is - Current through the LED for the given series resistor R.