If accuracy is important, consider using a current drive circuit (i.e. constant current source) to power each LED rather than the basic method of placing a resistor in series with a voltage source. You can find a basic current source [here][1]. If you use a current source you won't need to worry about variations in the forward voltage and temperature affecting the current in the LED.

If a constant current source is too complicated, you can approximate the forward characteristic of the diode using a pieceweise linear model, ie. a forward voltage plus a resistance. See the example [here][2]. From this it should be straighforward to get the current at 100mA.


  [1]: http://www.projectsbykec.com/projects/electronics/simple-current
  [2]: http://en.wikipedia.org/wiki/Diode_modelling#Diode-resistor_circuit_example