As well all know, using an LED without some sort of current control is a bad idea. One of the most basic types is a simple Ballast Resistor, paired with Ohm's Law. Based on Vin, Vf, and the desired Current, we can determine the resistor needed.
But in broad strokes, what is the best scenario for choosing a resistor? Given an LED with 20mA at 3.3V Forward Voltage, with a very close (Variable) source voltage, how much headroom does the resistor need for optimal control?
If we provide too much headroom (Vin of 12V) for a single 3.3V LED, we are just wasting energy and require a higher wattage resistor. If we provide little head room (Vin of 3.4V), we give the LED only 0.1V, is that enough? Changes in Vf and If due to temperature look like it would cause a looping cause and effect situation.
So the question is, for optimal control, how much headroom in terms of voltage, should the resistor need?