Grumpy_Mike:
This is why controlling LEDs with resistors is a bad idea,
It is not a bad idea it is done all the time in a lot of professional applications and is quite satisfactory technique. It is not the technique to get the absolute exact control over the current but often this is not necessary.
If you're aiming at 10-12mA (or less) on a 20mA LED, sure. The power curve is usually quite flat there.
If you're aiming at 20mA on a 20mA LED then resistors simply don't work.