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. Then you won't need to worry about the knowing the exact forward voltage of the LED. Incidently the VI curves for LEDs are temperature dependent.

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. From this it should be straighforward to get the current at 100mA.

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. 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. From this it should be straighforward to get the current at 100mA.