This is a problem I've never really got a proper handle on until today, helping one of the students at DJCAD: working out the appropriate resistor for an LED.
Having looked up the datasheet for the LED in question, applied 3 volts and promptly blown it up, I had a look for some more info and found this formula here:
The resistor value, R is given by:
|R = (VS - VL) / I|
VL = LED voltage (usually 2V, but 4V for blue and white LEDs)
I = LED current (e.g. 10mA = 0.01A, or 20mA = 0.02A)
Our supply for the radios is 3V, so I made an assumption and looked up a similar LED on Rapid, where we get our components. The spec showed it had a rated voltage of approx. 2V and current of 30mA (0.03A). Hence we need a resistor of 33.3 Ohms. We didn't have them so tried a 27 Ohm, which appears to work like a dream, although we held it arms length for a while to make sure it didn't blow up again!