General formula for LED resistor is ...
\$\huge R = \frac{V_{Supply} - V_{LED}}{I_{LED}}\$
Where:
- \$I_{LED}\$ is your chosen LED drive current which should be less than the maximum value stated in the LED's spec sheet.
- \$V_{LED}\$ is the minimum forward voltage of the LED at your chosen \$I_{LED}\$
However, it is not clear to me what this has to do with POTs and your interpretation of the lifespan on the LED. If you are intimating you want the LED to be driven with exactly 18mA, then you are better to drive the LED with a controlled current source instead of a simple resistor.