Question about resistors and LEDs
Chances are that for an perceived "half as bright" LED you'll need to increase the resistor by around  (5:1), which will yield around 20% of the lumens. 

Compare the popular North American auto tail light bulb the 1157, which has two filaments- brake and running light. The filaments are about 28W and about 8W, but the lumens are about 10:1 (402 vs. 38 lumens). Yet the brake lights are not perceived as ten times brighter- maybe 3 times. 

This is because your eye has approximately a logarithmic response over some range, so in fact a difference of 20% or 25% in lumens is barely perceived.