Is my voltage divider calculation OK? Can I use it to power an LED?
You can't use a voltage divider to power a load. That will almost never work.

You have two options:
 - Step down 20V to 12V using a LDO or switching regulator
 - Get your 12V out of the resistor network in an op-amp buffered with a NPN transistor

This describes accurately the problem with voltage dividers: http://www.learningaboutelectronics.com/Articles/Voltage-follower