What is your supply voltage? Most just play around with some to get it the way they want. Also if your supply changes with load consider that. I don't know if 1v would give half light output, I doubt it.
Have not checked the voltage, but should be under 4v I cant change the power supply as the Amp is not a DIY, its a commercial amplifier. Blocking part of the light is also not possible for aesthetics reasons, so a resistor in series is my only option
10ma is about 220 ohm. 5ma would be 430 according to a calculator on the net at 4v supply and 2v LED. So I guess I would get a 220 to 430+ ohm pot if possible, if the max current is 10ma. Or you know, put a 220 ohm on it and then a pot from 0 up to dim it. http://www.luxeonstar.com/resistor-calculator.php