The voltage drop across the LED doesn't vary much when the current flowing through it changes, it doesn't bahave like a resistor. Different colour LEDs have different voltage drops and are typically as follows: infrared 1.2V, red 1.8V - 1.9V, orange 1.9V - 2V yellow 2.1V, green 2.2V- 3.5V, blue/white 3.5, and voilet-ultravoilet 3.5V-4.2V, it's a good idea to look at the datasheet.
Calculating the series resistor value is fairly straight forward, you simply subtract the voltage drop from the supply then use ohm's law, as follows:
R = (Vsupply - VLED)/ILED