Voltage and Amperage for LED Cube
In order to determine the resistor value, you subtract the LED forward voltage (Vf), in volts,  from the supply voltage, in volts, then divide the difference by the LED forward current (If), in amperes. The quotient will be the desired resistance, in ohms.

In order to make sure you don't overcurent the LED use the minimum value of Vf from the data sheet.

In order to determine the required power rating for the resistor, multiply the difference between the supply voltage and Vf by If. The product will be the power the resistor will dissipate, in watts, and good practice dictates that to minimize the resistor's temperature rise a wattage higher than that be used.  I like to go 2:1 unless, for some extraordinary reason, I can't.