I say use a 5k potentiometer. While the diode thing has its advantages, you will want the ability to adjust it easily.

TS wants to drop somewhere between 2 and 3 volts at somewhere between 100 mA and 50 mA. A 5K pot is MUCH too high and will likely burn out at a very low setting. 3V/.1A = 30 ohms. 3V/.05A = 15 ohms. TS should start with a 30 ohm half watt resistor and be prepared to lower it with smaller, higher ohm resistors, such as 100 ohms. This assumes a reasonably well stocked resistor collection. Or, lacking such a collection, use the diodes.

 

Sorry. I meant more like a 500ohm potentiometer. That way you still have a wide range for speed.