I would try adding a resistor (say 500Ω) in series with one of the segments and slowly increase the voltage until you get the desired brightness. Then measure the voltage across the resistor and calculate the current (or connect an ammeter in series with the resistor to find the current). I expect it will be somewhere around 10-20mA. From that you can calculate the required resistor value for the voltage supply you have.
If you go too high in current the brightness will seem to stop increasing with current. At that point you would be overdriving the LED.