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First lets find the Resistor for 6 LEDS and see how that works. so supply is 12V Forward current is 20mA voltage drop is 1V, You have 12-6=6 So Ohm's law says R=E/I that means R=6/.02 or R=300 A 300 Ohm Resistor isn't common (Some thing the LED Calc takes into account) and most Resistors (at Radio shack) are 5% which mean they can be high or lower then rated be 5% so the 330 is a good choice.

This is the conservative method, your LEDs Should light and have a long life, but not be quite as bright (some times you can't tell the difference in brightness between this method and the over current method).

Now that I've muddy the water even more I'll get off my soap box

If you have any questions about what I've posted please let me know...I'll see if we can straighten them out.

CrazyBillyBob