Here is a good guide about LED's and resistors that was sent to me by Dan Dillon from the Cozy list:


Calculate The Resistor Value Needed: Assuming an aircraft nominal voltage of 13.8Vdc (engine running, battery
under charge), we need to drop around 10.3V across the resistor such that
the LED will have 3V dropped across it.  (We can think of an LED as a device
that sort of "uses up or drops" a certain amount of voltage during its
operation and we can calculate the resistor value by looking at what is
left.)

We need to know the current draw that the LED likes to see.  If we assume
20mA (0.020 Amps) as a reasonable current consumption value for an indicator
blinking LED, then we can use Ohm's Law to calculate the resistor
requirement:  Resistance = Voltage / Current

Resistance = Voltage across the resistor  / current through the resistor

Resistance = 10.3 volts / 20 mA

Resistance, in Ohms = 10.3/0.020

Resistance = 515 Ohms

(Minor Science Content:  The "voltage across the resistor" is equal
to the total voltage [battery] minus the voltage dropped across the LED.  We
know that the LED will drop 3V based on the datasheet for the product.  The
current through the resistor will be the same as the current through the
LED.  The current through each device in a series circuit is equal whereas
the voltage across each device is divided up in a way that it adds up to the
source [or battery] voltage.)