Alternatively, you could feed your leds (all together) from a constant current source, ensuring that they would always be the same voltage (and light output) regardless of current. An LM317 and a single resistor makes a nice
constant current regulator, at a cost of less than $2 and the arithmetic isn't hard (current output is 1.2/R1). Depending on your supply voltage you may need a small heatsink as well. FWIW the LM317/single resistor constant current design also makes a good constant current charging source for Nicads. BTW the 6V plugpack you have could have a voltage as high as 9-10 volts unless it's regulated.