This handy little tester can help you test LEDs or determine the correct resistor for the brightness you desire. It has a range of 2 mA to 160 mA.
To test an LED, you insert it into the holes for the desired milliamps. The holes are in 2 mA increments in the first row for a range of 2 mA to 32 mA. The second row has 5 mA increments for a range of 20 mA to 160 Ma.
You can also use the tester to test LEDs that are in engines or on wire leads by clipping test leads to wires or LED leads that are inserted into the holes. A Very handy device.
Once you have the correct mA, use this formula to find the resistor needed.