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If it's a small 3mm or 5mm LED, 20mA (and then measure the voltage at 20mA). FOr smaller LEDs, maybe 5 to 10mA, although many are fine for 20mA.
For high power LEDs (i.e. they are designed to operate on a heatsink), measure the voltage at 20mA then use this voltage to determine the current for 1W dissipation (Assuming it's a 1W or larger LED -- there's not too many that are smaller).
How do you determine the power rating of a high power LED -- that can be hard.
Remember never to power the LED from a voltage source. The voltage across the LED is only useful for calculating the value of a series resistor for low power LEDs, or to calculate the minimum voltage required for the drive circuit.
I'm not so sure of the 80% of the failure rating.
If you have a look at the life of LEDs vs. temperature (which is pretty closely related to current at a given ambient temperature) this method is likely to result in a much shortened lifetime.
Also, with a LED designed for a heatsink, do you do this with it attached to a heatsink? If so, how large?
You might decide that the maximum current you use gives you a Tj of 80C (for example) but this is pretty hard to measure.
To apply 20mA, I would place a 220 ohm resistor in series with the LED and connect it up to a variable power supply, initially set to whatever minimum voltage it has (less than 3V preferably)
I would then wind the voltage up slowly until one of the following occurs:
1) you measure 20mA
2) the voltage hits 5V and there is no measurable current
In the former case, measure the voltage across the LED.
In the latter case, you have the LED around the wrong way (turn the voltage back down, reconnect it and try again).
If you don't have a power supply with a variable output voltage then you can get a 6V battery pack and several resistors. 330, 270, 220, 180, 150, 120, and 100 ohms. Start with the 330 ohm resistor. If you have no current, turn the LED around. Once you see current, keep trying resistors until you get close to 20mA. Then measure the voltage across the LED.
You'll need a multimeter to do this, two would be even easier (unless your power supply can display current accurately)
If you're connecting several LEDs in parallel, each should have their own resistor.
You can find more information here: https://www.electronicspoint.com/got-question-driving-leds-t256849.html