It’s straightforward, but you should use the method described in the article: Start with the current and voltage, and work forward to the resistor that you need.
The mAh rating of a battery should only come in as a secondary consideration. That rating simply means how much energy is stored. If you are using two C cells in series, each rated for 8000 mAh, then you have 3 V total, with a capacity of 8000 mAh. Thus, if you were drawing 1 mA through the batteries to your LED, then it would run for 8000 hours.
