I usually use this method:
(derived from R=E/I)
R=(V-Vf)/I
R=Resistor Needed
V=Voltage applied to the resistor and LED
Vf is the forward voltage of the LED.
I is the current used by the LED (typically 20mA for red.) Remember, I is in Amps, so you must convert the LED's req'd mA into A.
An LED is a current device, not a voltage device. It will always drop the voltage by Vf. Adding more current will make them brighter.
